Control mechanism for stepless transmission

ABSTRACT

A control mechanism for a stepless transmission is a control mechanism connected to a manipulation lever in a stepless transmission and disposed outside a housing of the transmission. The control mechanism includes: a) a piston rod connected to the manipulation lever; b) a piston provided on the piston rod coaxially with the piston rod; c) a cylinder case provided with a cylinder adapted to house the piston rod and the piston such that the piston rod and the piston are displaceable in an axial direction, the piston and the cylinder forming a first fluid chamber to be supplied with a hydraulic fluid for withdrawing the piston rod from the cylinder and a second fluid chamber to be supplied with the hydraulic fluid for introducing the piston rod into the cylinder; d) a spring adapted to bias the manipulation lever in a neutral direction coaxially with the piston rod; e) a proportional pressure control valve adapted to selectively supply the hydraulic fluid to the first fluid chamber or the second fluid chamber, the proportional pressure control valves being mounted to the cylinder case; and f) a pivot shaft adapted to support the cylinder case oscillatably with respect to the housing.

CROSS-REFERENCE TO RELATED APPLICATIONS

The present application claims priority to Japanese Patent ApplicationsNo. 2014-161495, filed on Aug. 7, 2014, and No. 2014-162729, filed onAug. 8, 2014. Further, the present application is a continuation-in-partapplication of U.S. patent application Ser. No. 15/650,358, filed onJul. 14, 2017.

BACKGROUND OF THE INVENTION Field of the Invention

The present invention relates to a control mechanism for controlling theoutput rotary speed and direction of a hydrostatic transmission(hereinafter, “HST”). Especially, the control mechanism is aservomechanism including a servo unit that combines a telescopic(linearly movable) actuator and a valve.

Related Art

As disclosed by JP 2002-250437 A (hereinafter, “D1”), JP H06-100278 B(hereinafter, “D2”), and JP 2013-096449 A (hereinafter, “D3”), an HSTused as a transmission for a tractor is usually disposed in a casingthat also serves as a vehicle body frame (chassis) of the tractor. Also,as disclosed by D1, D2 and D3, it is well-known that a piston in ahydraulic cylinder serves as the actuator for controlling a position ofa movable swash plate of the HST. In this regard, a trunnion shaftserving as a pivot of the movable swash plate is connected to thepiston. Especially, each of the hydraulic cylinders disclosed by D2 andD3 is configured as a servomechanism.

With regard to connection of the actuator to the trunnion shaft, in D1,the trunnion shaft has a tip projecting outward from the casing, and anarm is fixed on the tip of the trunnion shaft, and is connected via alink to a tip of a piston rod extended from the hydraulic cylinderdisposed outside of the casing. Similarly, in D2, an arm is fixed on atip of the trunnion shaft projecting outward from the casing. However, atip of the arm is directly connected to the piston in the hydrauliccylinder without a link. In D3, a part of the casing serves as a housingof the hydraulic cylinder. The piston in the hydraulic cylinder formedin the casing is directly connected to the movable swash plate of theHST in the casing so that the HST and the hydraulic cylinder areassembled together in the casing.

To achieve a simple and compact vehicle body frame, some tractor makersdesire the HST to be minimized for its arrangement in the casing servingas the vehicle body frame, and the actuator to be separated from the HSTso as to enable its external attachment on the outside of the casing.From this viewpoint, the structure disclosed by D3 does not meet thesedesires.

If it is premised that the HST is disposed inside of the casing, and theactuator outside of the casing, it is desirable in assembility,maintenanceability, and reduction of parts count and costs that theactuator can be easily connected or disconnected to and from the tip ofthe trunnion shaft projecting outward from the casing. A space under thestep of the tractor on a right or left side of the casing is suggestedas one of appropriate spaces for arrangement of such an actuator.However, if the actuator is to be disposed under the step, the actuatormust be disposed at a considerably low position so that a sufficientvertical gap between the actuator and the step above the actuator isensured to facilitate works for connecting or disconnecting the actuatorto and from the trunnion shaft, and to prevent the attached actuatorfrom interfering with the step. Such a low position becomes considerablylower than the trunnion shaft. Therefore, as taught or suggested by D1and D2, a link or an arm is needed to connect the trunnion shaft to theactuator.

If the actuator is a telescopic actuator, such as a piston rod of ahydraulic cylinder, the movement of the actuator is linear while themovement of the arm that rotates centered on the trunnion shaft or thelike is circular. Therefore, such a differential movement between thearm and the actuator should be considered when they are connected toeach other. This differential movement becomes larger as the distancebetween the trunnion shaft and the actuator becomes larger. In otherwords, if a large vertical gap between the actuator and the trunnionshaft is desired to ensure the facility in work for connecting anddisconnecting the actuator to and from the trunnion shaft, there shouldbe any configuration for absorbing the differential movement between thearm and the actuator that may become large because of the large verticalgap.

In this regard, D1 discloses a structure that the cylinder serving asthe actuator is pivotally connected at the tip of the piston rod thereofto the arm, and at a cylinder bottom thereof to a part of the vehicle(e.g., a frame). During the telescopic movement of the piston rod, theentire cylinder rotates centered on the pivot at the cylinder bottomaccording to rotation of the arm. D2 discloses a structure that a grooveor a slot is provided in a portion of the piston of the servomechanismto the tip of the arm so as to absorb the rotation of the tip of the armduring the sliding movement of the piston.

However, in the case of D1, the part of the vehicle pivotally supportingthe cylinder bottom is weighed on or loaded eccentrically because itcantilevers the cylinder, so that it is liable to a twisting stress dueto the rotation of the cylinder. If the actuator is assembled with avalve so as to constitute a servo unit, especially, if the valve is alarge and heavy solenoid valve, the part pivoting the cylinder bottomfurther tends to be twisted. In the case of D2, the portion of thepiston engaged with the tip of the arm is complicated in structure. Itneeds accurate dimensions to surely absorb the differential movementbetween the piston and the arm while ensuring a stable telescopicmovement of the piston and a stable rotation of the arm. Further, theslot or groove has to be formed. As a result, the cost of theconfiguration of D2 becomes great. Moreover, in the case of D2, theactuator is a twin rod piston. While the piston engages with the tip ofthe arm, the actuator unit including the valve and the piston has to besupported by attaching both tips of the piston rods to the casing of theHST, thereby increasing the number of positions and processes forattaching or detaching the actuator unit.

Moreover, it is desirable that the servo unit is minimized as much aspossible. However, the actuator needs a biasing means that biases theactuator to a position corresponding to a neutral position of themovable swash plate, i.e., a neutral position of the HST. The existenceof the biasing means hinders the servo unit from being minimized.

SUMMARY OF THE INVENTION

An object of the invention is to downsize a servo unit and further tofacilitate attachment and detachment of the servo unit, in the casewhere an HST control mechanism such as a servo mechanism to be providedseparately from an HST includes the servo unit constituted by thecombination of an actuator and valves.

A control mechanism for stepless transmission according to the inventionis a control mechanism connected to a manipulation lever in a steplesstransmission and disposed outside a housing of the transmission, andincludes: a) a piston rod connected to the manipulation lever; b) apiston provided on the piston rod coaxially with the piston rod; c) acylinder case provided with a cylinder adapted to house the piston rodand the piston such that the piston rod and the piston are displaceablein an axial direction, the piston and the cylinder forming a first fluidchamber to be supplied with a hydraulic fluid for withdrawing the pistonrod from the cylinder and a second fluid chamber to be supplied with thehydraulic fluid for introducing the piston rod into the cylinder; d) aspring adapted to bias the manipulation lever in a neutral directioncoaxially with the piston rod; e) a proportional pressure control valveadapted to selectively supply the hydraulic fluid to the first fluidchamber or the second fluid chamber, the proportional pressure controlvalves being mounted to the cylinder case; and f) a pivot shaft adaptedto support the cylinder case oscillatably with respect to the housing.

Thus, the servo unit is pivotally supported through the pivot shaft,which can ensure the integrity of the servo unit constituted by thecombination of the actuator and the valves, can enhance theassemblability, and also can ensure easiness of attachment anddetachment of the servo unit.

Preferably, in the control mechanism for stepless transmission, theproportional pressure control valve is composed of a first valveconfigured to supply the hydraulic fluid to the first fluid chamber anda second valve adapted to supply the hydraulic fluid to the second fluidchamber, and an axis center of the piston rod, an axis center ofdisplacement of a valve body of the first valve and an axis center ofdisplacement of a valve body of the second valve form a triangle shape,when viewed in the axial direction of the piston rod.

This can reduce the radial size of a portion of the servo unit where theactuator and the valves are disposed, which can downsize the hydraulictransmission control mechanism, thereby preventing it from interferingwith obstructions existing around the housing.

Further, preferably, in the control mechanism for stepless transmission,the spring is arranged in the axial direction such that a coil axiscenter of the spring and the axis center of the piston rod aresubstantially coincident with each other.

This can reduce the radial size of a portion of the servo unit where theneutral spring is disposed, which can downsize the hydraulictransmission control mechanism, thereby preventing it from interferingwith obstructions existing around the housing.

Further, preferably, in the control mechanism for stepless transmission,the piston rod, the first valve, and the second valve are disposed suchthat the axial direction of the piston rod, a direction of displacementof a valve body of the first valve and a direction of displacement of avalve body of the second valve are parallel with each other, and thespring is disposed on one end of the cylinder case in the axialdirection of the piston rod, and the first valve and the second valveare disposed on the other end of the cylinder case on an opposite sideto the spring, in the axial direction of the piston rod.

This can reduce the radial size of the servo unit constituted by theactuator, the valves and the neutral spring which are integrated, whichcan downsize the hydraulic transmission control mechanism.

Further, in the control mechanism for stepless transmission furtherincludes a mounting part having the pivot shaft such that the mountingpart is oscillatable and is prevented from being disengaged from thehousing, wherein the mounting part is detachably mounted to the cylindercase.

This enables easily coping with changes of the specifications of thecontrol mechanism for stepless transmission, through replacement of themounting part.

Further, preferably, in the control mechanism for stepless transmission,the cylinder case is mounted to the mounting part such that an axis lineof the piston rod and an axis line of the oscillatable shaft portion areoverlapped with each other in a plan view.

This allows the cylinder case to smoothly oscillate during stretchingand shrinking of the piston rod, thereby efficiently moving themanipulation lever.

Further, preferably, in the control mechanism for stepless transmission,the piston rod is provided with a thread portion at a tip end portion,is secured, through screwing, to the manipulation lever connectingportion, and is provided with a flat portion in at least a portion of anouter peripheral surface.

This enables changing the position of the manipulation lever connectingportion by rotating a tool fitted to the flat portion, thereby easilyand accurately adjusting the neutral position.

These and other objects, features and advantages of the invention willappear more fully from the following detailed description of theinvention with reference to the attached drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a side view of a vehicle frame casing 11 to which a servo set40 according to a first embodiment is attached.

FIG. 2 is a sectional side view of vehicle frame casing 11 showing astructure of an HST 1 therein.

FIG. 3 is a perspective view of a servo unit 60.

FIG. 4 is a side view of servo unit 60.

FIG. 5 is a cross sectional view taken along V-V line of FIG. 4.

FIG. 6 is a cross sectional view taken along VI-VI line of FIG. 4.

FIG. 7 is a cross sectional view taken along VII-VII line of FIG. 6.

FIG. 8 is a cross sectional view taken along VIII-VIII line of FIG. 4.

FIG. 9 is a cross sectional view taken along IX-IX line of FIG. 4.

FIG. 10 is a cross sectional view taken along X-X line of FIG. 4.

FIG. 11 is a cross sectional view taken along XI-XI line of FIGS. 6 and7.

FIG. 12 is a sectional side view of a neutral returning unit 80.

FIG. 13 is a hydraulic circuit diagram for supplying fluid to HST 1 andservo set 40.

FIG. 14 is a sectional side view of HST 1 disposed in vehicle framecasing 11 to be provided with a servo unit 30 according to a secondembodiment.

FIG. 15 is a cross sectional view taken along XV-XV line of FIG. 14 as asectional rear view of HST 1 provided with servo unit 30 according tothe second embodiment.

FIG. 16 is a sectional side view of a portion of servo unit 30 includinga hydraulic cylinder 32.

FIG. 17 is a hydraulic circuit diagram for supplying fluid to HST 1 andservo unit 30.

FIG. 18 is a sectional rear view of a servo unit 30A.

FIG. 19 is a sectional rear view of a servo unit 30B.

FIG. 20 is a sectional rear view of a servo unit 30C.

FIG. 21 is a sectional rear view of a servo unit 30D.

FIG. 22 is a sectional rear view of a servo unit 30E.

FIG. 23 is a sectional side view of servo unit 30E.

FIG. 24 is a hydraulic circuit diagram for supplying fluid to HST 1 anda servo unit 90.

FIG. 25 is a side view of servo set 200.

FIG. 26 is a rear view of servo set 200.

FIG. 27 is a front view of servo set 200.

FIG. 28 is a perspective view of servo set 200.

FIG. 29 is a cross-sectional view taken along A-A line of FIG. 25.

FIG. 30 is a cross-sectional view taken along B-B line of FIGS. 25 and26.

FIG. 31 is a cross-sectional view taken along C-C line of FIG. 26.

FIG. 32 is a cross-sectional view taken along D-D line of FIG. 26.

FIG. 33 is a side view illustrating a state of mounting of servo set 200to vehicle frame casing 11.

FIG. 34 is a perspective assembly view illustrating the state ofmounting of servo set 200 to vehicle frame casing 11.

FIG. 35 is a front-portion cross-sectional view illustrating a portionfor pivotally supporting servo set 200 on vehicle frame casing 11.

FIG. 36 is a hydraulic circuit diagram for supplying fluid to HST1 andservo set 200.

DETAILED DESCRIPTION OF THE INVENTION

A first embodiment about an HST control mechanism (servomechanism)serving as an actuator device for controlling a movable swash plate ofan HST (Hydro-Static-Transmission) shown as an example of a continuouslyvariable transmission capable of switching forward and reverse will bedescribed with reference to FIGS. 1 to 13. Referring to FIGS. 1 and 2,an HST 1, a vehicle frame casing 11 and an axle casing 12 will bedescribed. Vehicle frame casing 11 is a casing formed to serve as avehicle body frame (chassis). Vehicle frame casing 11 incorporates HST 1serving as a unit for changing a speed of a vehicle such as a tractor.Vehicle frame casing 11 has front and rear open ends. One of the frontand rear end open ends of HST 1 (in this embodiment, the front open end)is an open end 11 a, which is joined to an end portion (in thisembodiment, a rear end portion) of an engine 10 (see FIG. 13). The otherof the front and rear ends of HST 1 is an open end 11 b, which is joinedto an end portion (in this embodiment, a front end portion) of axlecasing 12. Hereinafter, positions and directions of all componentelements of the first embodiment and a later-discussed second embodimentare defined on the assumption that vehicle frame casing 11 has frontopen end 11 a and rear open end 11 b.

Axle casing 12 journals an axle (not shown) and incorporates atransmission for driving the axle. This transmission transmits poweroutputted from HST 1 to the axles. The transmission may include aspeed-changing mechanism (e.g., a gear transmission). In this case, HST1 serves as a main speed-changing transmission, and the transmission inaxle casing 12 serves as a sub speed-changing transmission.

Vehicle frame casing 11 and axle casing 12 joined to each other serve asa vehicle body frame, which is disposed at a laterally middle portion ofa vehicle such as a tractor, for example. Referring to FIG. 1 (and FIG.2 illustrating another embodiment), this vehicle body frame is fixedlyprovided on right and left ends thereof with left and right steps 13 and14 made of horizontal plates, which are used for an operator's gettingon and off the vehicle, and as a foot rest for an operator sitting on aseat. A servo set (servomechanism) 40 for controlling a later-discussedmovable swash plate of HST 1 is disposed under one of steps 13 and 14(in this embodiment, step 14. See FIG. 15 or others illustrating thesecond embodiment).

HST 1 includes a hydraulic pump 2, a hydraulic motor 3, a center section4, an HST housing 5, a charge pump 20 and others, which are assembledtogether into a unit defined as HST 1. In this embodiment, centersection 4 is shaped as a vertical plate, which is fixed in vehicle framecasing 11 so as to cover rear open end 11 b of vehicle frame casing 11.A vertical front surface of center section 4 is formed with a pumpmounting surface and a motor mounting surface, which are verticallyjuxtaposed so that one is above the other. In this embodiment, the pumpmounting surface is disposed above the motor mounting surface. Acylinder block 2 a of hydraulic pump 2 is rotatably slidably mountedonto the pump mounting surface, and a cylinder block 3 a of hydraulicmotor 3 onto the motor mounting surface. A valve plate may be interposedbetween each cylinder block 2 a or 3 a and the pump or motor mountingsurface. HST housing 5 is extended forward from center section 4 so asto cover hydraulic pump 2 and hydraulic motor 3 at upper, lower, right,left and front sides of hydraulic pump 2 and motor 3.

Referring to FIG. 2, a front wall of HST housing 5 is disposed forwardfrom hydraulic pump 2 and motor 3. A movable swash plate 6 of hydraulicpump 2 is disposed adjacent to an upper portion of the front wall of HSThousing 5. Movable swash plate 6 is a trunnion-type movable swash platehaving later-discussed lateral trunnion shafts pivoted by right and leftside walls of HST housing 5, and abuts against heads (front ends) ofplungers 2 b projecting forward from cylinder block 2 a. Alater-discussed pump shaft 2 c is extended forward from movable swashplate 6, is journalled by an upper portion of the front wall of HSThousing 5, and is extended forward from HST housing 5 to as to bedrivingly connected to a later-discussed flywheel 10 b. On the otherhand, a fixed swash plate 6 of hydraulic motor 3 is fixed to a lowerportion of the front wall of HST housing 5 so as to abut against headsof plungers 3 b projecting forward from cylinder block 3 a.

Movable swash plate 6 includes right and left trunnion shafts serving asa fulcrum axis for rotation of movable swash plate 6. Right and lefttrunnion shafts are journalled by right and left walls of HST housing 5.Of the trunnion shafts, one trunnion shaft 6 b projects outward from HSThousing 5, and has an end portion projecting outward from a right orleft (in this embodiment, right) outer side surface 11 c of vehicleframe casing 11 just below step 14. A first arm 107 is fixed at a topportion thereof to the end portion of trunnion shaft 6 b. First arm 107is extended downward along the outer side surface of vehicle framecasing 11, and has a lower end portion to which a rear end portion 82 bof a rod 82 extended rearward from a cylindrical member 81 of a neutralreturning unit 80.

A second arm 109 is disposed rearward from first arm 107 outside ofvehicle frame casing 11, and is extended along outer side surface 11 cof vehicle frame casing 11. Second arm 109 has a pivot shaft 109 a at avertically intermediate portion thereof so as to be pivoted onto outerside surface 11 c of vehicle frame casing 11 via pivot shaft 109 a.Second arm 109 is pivotally connected at a top portion thereof to avertically intermediate portion of first arm 107 via a link 108. A tip(i.e., rear end) of a piston rod 33 a extended rearward from a hydrauliccylinder 32 of a servo unit 60 is pivotally connected to a lower endportion of second arm 109. Servo unit 60 and a neutral returning unit 80are assembled together so as to constitute a servo set (i.e.,servomechanism) 40 for controlling movable swash plate 6 of hydraulicpump 2. In this regard, first arm 107, link 108 and second arm 109 serveas a rotary member, to which a piston rod 33 a of a piston 33 serving asa telescopic actuator of servo unit 60 and a rear end portion 82 b of arod 82 serving as a telescopic member of neutral returning unit 80 arepivotally connected. Neutral returning unit 80 includes a neutralbiasing spring 83 (the neutral spring) serving as a biasing device.Neutral biasing spring 83 biases rod 82 toward its neutral positioncorresponding to a neutral position of movable swash plate 6.

Due to a fore-and-aft telescopic movement of piston rod 33 a of servounit 60, second arm 109 rotates centered on pivot shaft 109 a.Accordingly, first arm 107 connected to second arm 109 via link 108rotates centered on an axis of trunnion shaft 6 b, thereby rotatingmovable swash plate 6 of hydraulic pump 2, because trunnion shaft 6 b ofmovable swash plate 6 is fixed to first arm 107. Further, due to thetelescopic movement of piston rod 33 a and the rotation of second arm109, first arm 107 rotates so as to move rod 82 telescopically againstthe biasing force of neutral biasing spring 83. In servo unit 60,proportional pressure control valves 35 and 36 can be controlled toapply an operation force to piston rod 33 a. Once piston rod 33 a isreleased from the operation force, rod 82 returns to the neutralposition defined by the biasing force of neutral biasing spring 83,thereby returning first arm 107 and movable swash plate 6 to theirneutral positions. Due to the movement of first arm 107 to its neutralposition, second arm 109 is rotated via link 108 so that piston rod 33 areturns to a position in the telescopic movement direction (fore-and-aftdirection) of piston rod 33 a corresponding to the neutral position ofmovable swash plate 6 with trunnion shafts 6 a.

Hydraulic pump 2 includes a fore-and-aft horizontal pump shaft 2 cserving as a rotary axis of cylinder block 2 a. Hydraulic motor 3includes a fore-and-aft horizontal motor shaft 3 c serving as a rotaryaxis of cylinder block 3 a. Pump shaft 2 c and motor shaft 3 c arejournalled by center section 4. Rear end portions of pump shaft 2 c andmotor shaft 3 c project rearward from center section 4 into axle casing12. In axle casing 12, a front end portion of a PTO shaft or a front endportion of a PTO transmission shaft 15 interlocking with the PTO shaftis connected via a coupling 15 a to the rear end portion of pump shaft 2c. On the other hand, in axle casing 12, a front end portion of an inputshaft 16 of the transmission in axle casing 12 is connected via acoupling 16 a to the rear end portion of motor shaft 3 c serving as anoutput shaft of HST 1.

In vehicle frame casing 11, a gear casing 21 is fixed to a front end ofHST housing 5. A front portion of pump shaft 2 c projects forward fromthe front wall of HST housing 5 through gear casing 21. After engine 10is joined to front open end 11 a of vehicle frame casing 11, afore-and-aft horizontal engine output shaft 10 a of engine 10 isextended into vehicle frame casing 11 via front open end 11 a, so that aflywheel 10 b provided at a rear end portion of engine 10 is disposed ina front inside space in vehicle body frame 11. Pump shaft 2 c serving asan input shaft of HST 1 projects forward from gear casing 21 asmentioned above, so that a front end portion of pump shaft 2 c isconnected to flywheel 10 b via a damper 10 c.

In gear casing 21, pump shaft 2 c is journalled at a fore-and-aftintermediate portion thereof, and a charge pump driving shaft 24 isextended parallel to pump shaft 2 c and is journalled. In gear casing21, a gear 22 is fixed on the fore-and-aft intermediate portion of pumpshaft 2 c, and a gear 23 is fixed on charge pump driving shaft 24. Gears22 and 23 mesh with each other so as to constitute a gear train fortransmitting power from pump shaft 2 c to charge pump driving shaft 24.A pump housing 26 incorporating charge pump 20 is fixed to a front endof gear casing 21 so as to extend parallel to the front portion of pumpshaft 2 c projecting forward from gear casing 21. Charge pump drivingshaft 24 projects forward from gear casing 21 into pump housing 25. Inpump housing 25, a pump driven shaft 20 c parallel to charge pumpdriving shaft 24 is journalled. In pump housing 25, a pump gear 20 afixed on charge pump driving shaft 24 and a pump gear 20 b fixed on pumpdriven shaft 20 c mesh with each other so as to constitute a gear pumpserving as charge pump 20.

Referring to FIGS. 2 and 13, a hydraulic fluid supply system forsupplying fluid from charge pump 20 to HST 1 will be described. Chargepump 20 sucks fluid from a fluid sump in axle casing 12. In this regard,axle casing 12 has a fluid discharge port P1. Fluid of the fluid sump inaxle casing 12 is taken out of axle casing 12 via fluid discharge portP1 and is sucked to a suction port of charge pump 20 disposed in vehicleframe casing 11 via a fluid passage L1. Fluid passage L1 may be formedwithin a wall of vehicle frame casing 11, or may be made of pipesdisposed outside of casings 11 and 12, for example.

Fluid delivered from a delivery port of charge pump 20 is taken out ofvehicle frame casing 11 again, and is supplied into a charge fluidpassage 4 a formed in center section 4 via a line filter 26, a fluidpassage L2 and a charge port P2. In the embodiment shown in FIG. 2,charge fluid passage 4 a has an open end serving as charge port P2 at arear end surface of center section 4, and fluid passage L2 is connectedto charge port P2.

A relief valve 28 is provided in center section 4 so as to adjust ahydraulic pressure of fluid in charge fluid passage 4 a of centersection 4. Fluid released from relief valve 28 is drained into a chamberin HST housing 5 forward from center section 4 via a drain fluid passage4 b formed in center section 4. Referring to FIG. 1, HST housing 5 has adrain port for draining fluid from the inside of HST housing 5. Thisdrain port is normally plugged with a drain cap 29, as shown in FIG. 2.Fluid from the drain port is drained to the fluid sump in HST housing 5or vehicle frame casing 11. This fluid sump may be fluidly connected toa fluid sump in axle casing 12 via a fluid passage L5, as shown in FIG.13.

Referring to FIG. 13, center section 4 is formed therein with a pair ofmain fluid passages ML1 and ML2 that are interposed between hydraulicpump 2 and hydraulic motor 3. Center section 4 is provided therein witha pair of charge valve units CV1 and CV2. The fluid in charge fluidpassage 4 a has a hydraulic pressure regulated by relief valve 28. Whenmain fluid passage ML1 is hydraulically depressed, a check valve 45 incharge valve unit CV1 is opened so that the fluid in charge fluidpassage 4 a is supplied to main fluid passage ML1 via opened check valve45 in charge valve unit CV1. When main fluid passage ML2 ishydraulically depressed, a check valve 45 in charge valve unit CV2 isopened so that the fluid in charge fluid passage 4 a is supplied to mainfluid passage ML2 via opened check valve 45 in charge valve unit CV2.Each of charge valve units CV1 and CV2 includes a relief valve 46 thatbypasses corresponding check valve 45 so as to regulate the hydraulicpressure in corresponding main fluid passage ML1 or ML2.

One of charge valve units CV1 and CV2 includes an orifice 47 bypassingcorresponding check valve 45 so that orifice 47 functions to expand aneutral region of HST 1. If main fluid passage ML2 is designated to havea higher hydraulic pressure than that of main fluid passage ML1 duringbackward traveling of the vehicle, preferably, charge valve unit CV2 forsupplying fluid to main fluid passage ML2 has orifice 47.

Referring to the hydraulic circuit diagram of FIG. 13, a generalconfiguration of servo set 40 and a fluid supply system for supplyingfluid from charge pump 20 to servo unit 60 of servo set 40. A deliveryfluid passage from charge pump 20 passes line filter 26 and then isbifurcated into fluid passages L2 and L3. Fluid passage L2 is extendedinto HST 1. Fluid passage L3 is extended to servo unit 60. In this way,charge pump 20 supplies hydraulic fluid to both HST 1 and servo unit 60for controlling movable swash plate 6 of HST 1.

Servo unit 60 includes a hydraulic cylinder 32 and proportional pressurecontrol valves 35 and 36. Piston 33 serving as the actuator forcontrolling the position of movable swash plate 6 is slidably fitted inhydraulic cylinder 32. Opposite piston rods 33 a and 33 b are extendedfrom piston 33 and project outward from opposite ends of hydrauliccylinder 32. The tip of piston rod 33 a is pivotally connected to secondarm 109 as mentioned above.

Proportional pressure control valve 35 is adapted to supply or dischargefluid to and from a fluid chamber 32 a formed in hydraulic cylinder 32on one side of piston 33. Proportional pressure control valve 36 isadapted to supply or discharge fluid to and from a fluid chamber 32 bformed in hydraulic cylinder 32 on the other side of piston 33.Proportional pressure control valves 35 and 36 are proportional solenoidvalves provided with respective proportional solenoids 35 a and 36 a.Proportional pressure control valve 35 has a valve port 35 d fluidlyconnected to fluid chamber 32 a in hydraulic cylinder 32 via a hydraulicfluid passage 43. Proportional pressure control valve 36 has a valveport 36 d fluidly connected to fluid chamber 32 b in hydraulic cylinder32 via a hydraulic fluid passage 44. One of proportional pressurecontrol valves 35 and 36 is designated so as to be excited duringforward traveling of the vehicle, and the other of proportional pressurecontrol valves 35 and 36 so as to be excited during backward travelingof the vehicle. Either proportional pressure control valve 35 or 36,which is excited, supplies fluid from its valve port 35 d or 36 d tocorresponding fluid chamber 32 a or 32 b via corresponding hydraulicfluid passage 43 or 44.

Servo unit 60 has an inlet port P3 and an outlet port P4. Inlet port P3receives fluid delivered from charge pump 20 via fluid passage L3.Outlet port P4 is fluidly connected to the fluid sump in vehicle framecasing 11 via a fluid passage L4. Proportional pressure control valves35 and 36 have respective suction ports 35 b and 36 b and respectivedrain ports 35 c and 36 c. The fluid introduced into servo unit 60 fromfluid passage L3 via inlet port P3 is supplied to suction ports 35 b and36 b of respective proportional pressure control valves 35 and 36. Adrain fluid passage 42 extended to outlet port P4 collects fluid fromboth drain ports 35 c and 36 c of proportional pressure control valves35 and 36 so as to drain the collected fluid to the fluid sump invehicle frame casing 11 via outlet port P4 and fluid passage L4 outsideof servo unit 60.

Each of the proportional solenoid valves serving as proportionalpressure control valves 35 and 36 has a movable member such as a spool.Each of proportional solenoids 35 a and 36 a has a driving force todrive the movable member in the direction against the spring force(i.e., the biasing force of neutral biasing spring 83 of neutralreturning unit 80) and the hydraulic pressure, and controls the positionof the movable member so as to balance the movable member against thespring force and the hydraulic pressure, thereby controlling the flowand pressure of fluid through corresponding proportional pressurecontrol valve 35 or 36. The driving force of each of proportionalsolenoids 35 a and 36 a is proportional to a current (cutoff current)value applied on proportional solenoid 35 a or 36 a. Referring to FIG.13, each of proportional pressure control valves 35 and 36 isvibratorily shifted between a supply position S and a drain position Din correspondence to the current value applied on its proportionalsolenoid 35 a or 36 a.

Each of proportional pressure control valves 35 and 36, when it is setat supply position S, fluidly connects its valve port 35 d or 36 d toits suction port 35 b or 36 b so that fluid introduced to suction port35 b or 36 b via inlet port P3 is supplied to corresponding fluidchamber 32 a or 32 b. Each of proportional pressure control valves 35and 36, when it is set at drain position D, fluidly connects its valveport 35 d or 36 d to its drain port 35 c or 36 c so that fluidintroduced to valve port 35 d or 36 d from corresponding fluid chamber32 a or 32 b is supplied to corresponding fluid chamber 32 a or 32 b.The vibratory shift of each of proportional pressure control valves 35and 36 between supply position S and drain position D means a repeat ofalternate supply and drain of fluid to and from corresponding fluidchamber 32 a or 32 b. Due to such repeated supply and drain of fluid, ahydraulic pressure in corresponding fluid chamber 32 a or 32 b is set sothat piston 33 is stroked to a position where the hydraulic pressure isbalanced against pressure in the other fluid chamber 32 a or 32 b andthe biasing force of neutral biasing spring 83. Incidentally, acontroller (not shown) creates the current value in correspondence to anoperation degree of a speed control manipulator (not shown) such as apedal or a lever.

The description of servo set 40 with reference to FIG. 13 is ended. Now,configurations of servo unit 60 and neutral returning unit 80 in servoset 40 will be described in detail with reference to FIGS. 1 to 12. Asmentioned above, servo unit 60 and neutral return unit 80 are disposedoutside of vehicle frame casing 11 so as to extend along outer sidesurface 11 c of vehicle frame casings 11. Hereinafter, with regard tothe lateral direction of servo unit 60 and neutral returning unit 80, aside close to outer side surface 11 c is referred to a “laterallyproximal” side, and a side away from (opposite) outer side surface 11 cis referred to a “laterally distal” side.

Referring to FIGS. 1 to 11, the configuration of servo unit 60 will bedescribed. Servo unit 60 includes a bracket 61, a valve block 64 and afluid duct block 65. As shown in FIGS. 3 and 6, bracket 61 is aplate-shaped member, which is extended in the fore-and-aft directionalong outer side surface 11 c of vehicle frame casing 11. Bracket 61 hasa rear end portion 61 a and a front end portion 61 b, which are bent tothe laterally distal side. Referring to FIGS. 3, 6 and 8 to 10, asleeve-shaped boss 62 is fixed on a fore-and-aft middle portion ofbracket 61 between rear and front end portions 61 a and 61 b and isextended to the laterally proximal side. Referring to FIGS. 3 and 6,bracket 61 is formed therethrough with a boss hole 61 c, and boss 62 isformed therethrough with a boss hole 62 a.

Referring to FIG. 6, a pivot shaft 63 is passed at a laterally distalportion thereof through boss holes 61 c and 62 a. Pivot shaft 63 isinserted at a laterally proximal end portion thereof into vehicle framecasing 11 through outer side surface 11 c so as to be retained invehicle frame casing 11. In other words, bracket 61 is pivoted at thefore-and-aft middle portion thereof on vehicle frame casing 11 via theproximally distal portion of laterally horizontal pivot shaft 63projecting outward from outer side surface 11 c of vehicle frame casing11. Therefore, rear end portion 61 a and front end portion 61 b ofbracket 61 are able to vertically oscillate centered on pivot shaft 63.

Referring to FIGS. 3, 5, 6 and 10, valve block 64 is formed with astepped surface 64 a, which contacts rear end portion 61 a of bracket61. On the other hand, referring to FIGS. 1 and 3 to 7, fluid duct block65 contacts front end portion 61 b of bracket 61 at a front end surfacethereof. Referring to FIGS. 1 and 3 to 10, hydraulic cylinder 32 issandwiched between valve block 64 and fluid duct block 65. Fourconnection rods 66 are disposed so as to surround hydraulic cylinder 32,and are extended parallel to hydraulic cylinder 32. Four connection rods66 consist of a pair of right and left connection rods 66 abovehydraulic cylinder 32, and a pair of right and left connection rods 66below hydraulic cylinder 32. In other words, they consist of a pair ofupper and lower connection rods 66 on the laterally proximal side ofhydraulic cylinder 32 (hereinafter referred to as upper and lowerproximal connection rods 66), and a pair of upper and lower connect rods66 on the laterally distal side of hydraulic cylinder 32 (hereinafterreferred to as upper and lower distal connection rods 66).

Referring to FIG. 5, each connection rod 66 is a bolt having a rear endportion formed as a bolt head 66 a, and a front end portion formed as athreaded shaft portion 66 b. Each connection rod 66 is inserted forwardinto valve block 64. In this regard, upper and lower proximal connectionrods 66 are inserted into valve block 64 via rear end portion 61 a ofbracket 61 contacting stepped surface 64 a of valve block 64. Eachconnection rod 66 has front threaded shaft portion 66 b screwed throughfluid duct block 65. In this regard, upper and lower proximal connectionrods 66 have respective front threaded shaft portions 66 b furtherscrewed through front end portion 61 b of bracket 61 contacting thefront end surface of fluid duct block 65.

As mentioned above, four connection rods 66 are passed through valveblock 64 and fluid duct block 65. Upper and lower distal connection rods66 have respective bolt heads 66 a contacting a rear end surface ofvalve block 64. Upper and lower proximal connection rods 66 haverespective bolt heads 66 a contacting rear end portion 61 a of bracket61. Therefore, four connection rods 66 fasten valve block 64 and fluidduct block 65 sandwiching hydraulic cylinder 32 therebetween to eachother. Further, upper and lower proximal connection rods 66 fasten valveblock 64 and fluid duct block 65 to rear and front end portions 61 a and61 b of bracket 61. In this way, bracket 61, boss 62, hydraulic cylinder32, valve block 64, hydraulic block 65 and connection rods 66 areassembled together into servo unit 60, so that they are all rotatablycentered on pivot shaft 63. Connection rods 66 are not limited innumber, position and so on. The only thing required for connection rods66 is to be appropriate to assembling of hydraulic cylinder 32, valveblock 64 and fluid duct block 65 so that hydraulic cylinder 32, valveblock 64 and fluid duct block 65 are integrally rotatable centered onpivotal shaft 63.

Moreover, referring to FIGS. 1, 3 and 4 to 7, a fluid pipe 67 isdisposed above hydraulic cylinder 32 and is extended parallel tohydraulic cylinder 32 so as to be interposed between valve block 64 andfluid duct block 65, thereby ensuring a fluid flow between valve block64 and fluid duct block 65. A configuration of fluid passages, includingfluid pipe 67, will be detailed later.

Referring to FIGS. 6 and 7, in this embodiment, the chamber close to therear end of hydraulic cylinder 32 fixed to valve block 64 serves asfluid chamber 32 a, and the chamber close to the front end of hydrauliccylinder 32 fixed to fluid duct block 65 serves as fluid chamber 32 b.Piston rod 33 a is extended rearward from piston 33 in hydrauliccylinder 32 so as to pass through fluid chamber 32 a of hydrauliccylinder 32. Piston rod 33 a further passes through valve block 64 andprojects rearward from valve block 64 so as to have the rear end portionpivotally connected to the lower end portion of second arm 109 asmentioned above. On the other hand, piston rod 33 b is extended forwardfrom piston 33 in hydraulic cylinder 32 so as to pass through fluidchamber 32 b of hydraulic cylinder 32. Piston rod 33 b further passesthrough fluid duct block 65 and projects at a front end portion thereofforward from fluid duct block 65.

According to the fore-and-aft slide of piston 33 in hydraulic cylinder32, piston rod 33 a projecting rearward from valve block 64 and pistonrod 33 b projecting forward from fluid duct block 65 are movedtelescopically (linearly). Referring to FIGS. 1 and 13, the linearmovement of piston rod 33 a causes second arm 109 to rotate centered onpivot shaft 109 a. During the rotation of second arm 109, the lower endportion of second arm 109 changes its position in the verticaldirection, so that the rear end portion of piston rod 33 a pivotallyconnected to the lower end portion of second arm 109 is oscillatedvertically. The vertical oscillation of the rear end portion of pistonrod 33 a is enabled by the above-mentioned integral rotatability ofhydraulic cylinder 32, valve block 64 and fluid duct block 65 centeredon pivot shaft 63. In other words, according to the telescopic movementof piston rod 33 a, hydraulic cylinder 32, valve block 64 and fluid ductblock 56 are integrally rotated centered on pivot shaft 63 so as toprevent second arm 109 pivotally connected to piston rod 33 a from beingtwisted, whereby second arm 109 can rotate according to the telescopic(linear) movement of piston rod 33 a.

Valve block 64 further expands to the laterally distal side from theportion thereof connected to hydraulic cylinder 32 and connection rods66. The laterally distal expanded portion of valve block 64 incorporatesproportional pressure control valves 35 and 36 that are juxtaposed upperand lower. In this embodiment, proportional pressure control valve 36fluidly connected to fluid chamber 32 b is disposed above proportionalpressure control valve 35 fluidly connected to fluid chamber 32 a.Proportional solenoids 35 a and 36 a of proportional pressure controlvalves 35 and 36 project forward from the expanded portion of valveblock 64 and are extended parallel to hydraulic cylinder 32.

Referring to FIGS. 6, 9 and 11, valve block 64 is bored therein with avertical fluid hole 71. In valve block 64, fluid hole 71 is fluidlyconnected at a vertically intermediate portion thereof to suction port36 b of proportional pressure control valve 36, and at a lower endportion thereof to suction port 35 b of proportional pressure controlvalve 35. Fluid hole 71 serves as suction fluid passage 41 shown in thehydraulic circuit diagram of FIG. 13. Referring to FIGS. 3, 9 and 11, anupper end 71 a of fluid hole 71 is open at a top surface of valve block64, so that upper end 71 a of fluid hole 71 serves as inlet port P3shown in the hydraulic circuit diagram of FIG. 13.

Referring to FIGS. 6 and 8, valve block 64 is bored therein with avertical fluid hole 72. An upper end 72 a of fluid hole 72 is disposedat the top surface of valve block 64 and is plugged. Fluid hole 72 isfluidly connected at a vertically intermediate portion thereof to drainport 36 c of proportional pressure control valve 36, and at a lower endthereof to drain port 35 c of proportional pressure control valve 35.Referring to FIG. 8, valve block 64 is further bored therein with ahorizontal fluid hole 73 that is extended laterally distally from avertically intermediate portion of fluid hole 72. Fluid holes 72 and 73formed in valve block 64 serve as drain fluid passage 42 shown in thehydraulic circuit diagram of FIG. 13. Referring to FIGS. 1, 3, 4 and 8,fluid hole 73 has a laterally distal end 73 a open at a laterally distalside surface of valve block 64, so that laterally distal end 73 a offluid hole 73 serves as outlet port P4 in FIG. 13.

Referring to FIGS. 6 to 10, valve block 64 is bored in a lower halfportion thereof with a vertically slant fluid hole 74 extended laterallyalong the rear end surface of valve block 64. Referring to FIGS. 3 to 6and 10, a laterally distal end 74 a of fluid hole 74 is disposed at thelaterally distal side surface of valve block 64, and is plugged. Fluidhole 74 is fluidly connected at a laterally intermediate portion thereofto valve port 35 d of proportional pressure control valve 35. Referringto FIGS. 6 and 8 to 10, valve block 64 is bored therein with afore-and-aft horizontal fluid hole 75. Referring to FIGS. 6 and 10, arear end of fluid hole 75 and a laterally proximal end of fluid hole 74are joined to each other. Referring to FIG. 6, a front end of fluid hole75 is open at a rear end of fluid chamber 32 a in hydraulic cylinder 32.In this way, fluid holes 74 and 75 formed in valve block 64 serve ashydraulic fluid passage 43 that fluidly connects valve port 35 d ofproportional pressure control valve 35 to fluid chamber 32 a ofhydraulic cylinder 32 as shown in FIG. 13.

Referring to FIGS. 7, 10 and 11, valve block 64 is bored in an upperhalf portion thereof with a laterally horizontal fluid hole 76 along therear end surface of valve block 64. Referring to FIGS. 3, 4 and 10, alaterally distal end 76 a of fluid hole 76 is disposed at the laterallydistal side surface of valve block 64, and is plugged. Valve hole 76 isfluidly connected at a laterally intermediate portion thereof to valveport 36 d of proportional pressure control valve 36. Further, referringto FIGS. 7 to 10, valve block 64 is bored therein with a fore-and-afthorizontal fluid hole 77. Referring to FIGS. 7 and 10, a rear end offluid hole 77 and a laterally proximal end of fluid hole 76 are joinedto each other. On the other hand, as mentioned above, fluid pipe 67 isinterposed between valve block 64 and fluid duct block 65. Referring toFIG. 7, a rear open end portion of fluid pipe 67 is fitted into valveblock 64 and is joined to a front open end of fluid hole 77.

Referring to FIG. 7, fluid duct block 65 is bored therein with avertical fluid hole 78. Referring to FIGS. 3 and 7, an upper end offluid hole 74 is disposed at the top surface of fluid duct block 65, andis plugged. A front open end portion of fluid pipe 67 is fitted intofluid block 65 and is joined to a vertically intermediate portion offluid hole 78. Fluid hole 78 is fluidly connected at a lower end thereofto fluid chamber 32 b via a front end portion of hydraulic cylinder 32fitted into fluid duct block 65. In this way, fluid holes 76 and 77formed in valve block 64, fluid pipe 67 interposed between valve block64 and fluid duct block 65, and fluid hole 78 formed in fluid duct block65 serve as hydraulic fluid passage 44 that fluidly connects valve port36 d of proportional pressure control valve 36 to fluid chamber 32 b ofhydraulic cylinder 32 as shown in the hydraulic circuit diagram of FIG.13.

Neutral returning unit 80 will be described in detail with reference toFIGS. 1, 12 and 13. Neutral returning unit 80 includes cylinder member81, rod 82, a neutral biasing spring 83, spring retainers 84, a bracket85, a pivot shaft 86, retaining rings 87, and a nut 88. As mentionedabove, cylindrical member 81 is extended along outer side surface 11 cof vehicle frame casing 11. Cylindrical member 81 is pivoted at afore-and-aft middle portion thereof on vehicle frame casing 11 via pivotshaft 86, so that cylindrical member 81 can be vertically oscillated atfront and rear ends thereof centered on pivot shaft 86 at thefore-and-aft intermediate portion thereof.

Referring to FIG. 1, bracket 85 surrounds the fore-and-aft intermediateportion of cylindrical member 81 pivoted on pivot shaft 86. Bracket 85is formed at upper and lower end portions thereof with respective tabs85 a having respective bolt holes 85 b. Bolts (not shown) are passedthrough respective bolt holes 85 b so as to fasten upper and lower tabs85 a of bracket 85 to outer side surface 11 c of vehicle frame casing11, thereby fixing bracket 85 to vehicle body frame 11. Bracket 85 isspaced from upper and lower end portions of cylindrical member 81 so asto allow cylindrical member 81 to oscillate centered on pivot shaft 86.Conversely, the space between bracket 85 and cylindrical member 81defines an oscillation range of cylindrical member 81.

Cylindrical member 81 is open at front and rear ends thereof. Rod 82 isfore-and-aft extended through cylindrical member 81. A rear end portion82 b of rod 82 is extended rearward from the rear end of cylindricalmember 81 and is pivotally connected to the tip of first arm 107, asmentioned above. Rod 82 is axially (fore-and-aft) slidable so that afront end portion 82 c of rod 82 is able to project forward from thefront end of cylindrical member 81 when rod 82 slides forward.

In cylindrical member 81, front and rear spring retainers 84 are fittedon rod 82 so as to be axially slidable relative to rod 82. Neutralbiasing spring 83 is interposed between front and rear spring retainers84 and is coiled to surround rod 82. Front and rear retaining rings 87are fixed on front and rear inner peripheral portions of cylindricalmember 81. Front retaining ring 87 restricts a forward slide of frontspring retainer 84. Rear retaining ring 87 restricts a rearward slide ofrear spring retainer 84.

FIG. 12 illustrates neutral returning unit 80 having neutral biasingspring 83 compressed in its initial state. In this state, due to thefore-and-aft expansion force of neutral biasing spring 83, both frontand rear spring retainers 84 are pressed against respective retainingrings 87. A flange 82 is formed on an axially intermediate portion ofrod 82 so that flange 82 a contacts rear spring retainer 84 contactingrear retaining ring 87. Nut 88 is screwed on rod 82 close to front endportion 82 c so that nut 88 contacts a front end of front springretainer 84 contacting front retaining ring 87.

A position of rod 82 in its sliding direction where neutral biasingspring 83 is in the initial state as shown in FIG. 12 is defined as aneutral position of rod 82 serving as the telescopic member. Theconfiguration to connect rod 82 to movable swash plate 6 via first arm107 is designed so that movable swash plate 6 is set at its neutralposition when rod 82 is disposed at the neutral position of rod 82. Onthe other hand, a position of piston 33 realized by the condition ofhydraulic fluid in fluid chambers 32 a and 32 b in hydraulic cylinder 32when both proportional solenoids 35 a and 35 b of proportional pressurecontrol valves 35 and 36 are unexcited is defines as a neutral positionof piston 33. The configuration to connect rod 82 to piston rod 33 a viafirst arm 107, link 108 and second arm 109 is designed so that piston 33is set at its neutral position when rod 82 is disposed at the neutralposition of rod 82.

When piston 33 with piston rods 33 a and 33 b slides forward from itsneutral position, this telescopic movement of piston 33 is transmittedto movable swash plate 6 via second arm 109, link 108 and first arm 107so as to tilt movable swash plate 6 in one direction for either forwardor backward traveling of the vehicle. At this time, the lower endportion of first arm 107 rotates rearward so that rod 82 slidesrearward. Cylindrical member 81 oscillates centered on pivot shaft 86 soas to absorb the differential movement between the circular movement offirst arm 107 serving as the rotary member and the linear movement ofrod 82 serving as the telescopic member. According to the rearward slideof rod 82, nut 88 fixed on rod 82 pushes front spring retainer 84rearward while rear spring retainer 84 is kept to contact rear retainingring 87. Therefore, neutral biasing spring 83 is compressed further fromthe initial compression state so as to apply a forward biasing force torod 82, thereby biasing rod 82, movable swash plate 6 and piston 33toward the respective neutral positions.

On the other hand, when piston 33 with piston rods 33 a and 33 b slidesrearward from its neutral position, movable swash plate 6 is tilted inthe other direction for either forward or backward traveling of thevehicle. At this time, the lower end portion of first arm 107 rotatesforward so that rod 82 slides forward. Cylindrical member 81 oscillatescentered on pivot shaft 86 so as to absorb the differential movementbetween the rotation first arm 107 and the linear movement of rod 82.According to the forward slide of rod 82, flange 82 a on rod 82 pushesrear spring retainer 84 forward while front spring retainer 84 is keptto contact front retaining ring 87. Therefore, neutral biasing spring 83is compressed further from the initial compression state so as to applya rearward biasing force to rod 82, thereby biasing rod 82, movableswash plate 6 and piston 33 toward the respective neutral positions.

As mentioned above, the neutral position of rod 82 defines the neutralpositions of first arm 107 and movable swash plate 6, and defines theneutral position of piston 33 (while both proportional solenoids 35 aand 36 a of proportional pressure control valves 35 and 36 areunexcited) via first arm 107, link 108 and second arm 109. In otherwords, when rod 82 of neutral returning unit 80 is returned to itsneutral position by the biasing force of neutral biasing spring 83,piston 33 of servo unit 60 also returns to its neutral position.

Referring to FIG. 6, in the fore-and-aft direction, the neutral positionof piston 33 also substantially coincides to the position of pivot shaft63 oscillatively supporting hydraulic cylinder 32, thereby balancing theaction of piston 33 in hydraulic cylinder 32, and thereby reducing theoscillation of hydraulic cylinder 32 according to the telescopicmovement of piston rod 33 a.

A second embodiment about a hydraulic servo unit serving as an actuatordevice for controlling a movable swash plate of an HST will now bedescribed with reference to FIGS. 14 to 24. Description of the membersand portions designated by the reference numerals used for descriptionof the first embodiment is omitted because they are identical or similarto the respective members and portions of the first embodimentdesignated by the same reference numerals.

An object of this embodiment is to provide an actuator unit that caneasily be connected to a movable swash plate of the HST, which needs nolink for its connection to an arm on a tip of a trunnion shaft of themovable swash plate, and which needs no work for fixing an arm to thetrunnion shaft or for attaching or connecting a hydraulic cylinder or apiston rod serving as the actuator to a part of a vehicle body, such asthe above-mentioned vehicle frame casing, thereby improving the HST withthe actuator unit in assembility and maintenanceability.

Referring to FIGS. 14 and 15, HST 1, vehicle frame casing 11 and axlecasing 12 are configured similar to those of the first embodimentdescribed with reference to FIGS. 1 and 2. The configurations of HST 1,vehicle frame casing 11 and axle casing 12, which have been omitted fromthe description of those of the first embodiment, or which are differentfrom those of the first embodiment, will be described in detail.Referring to FIG. 15, right and left coaxial through holes 5 b and 5 care formed in right and left side walls of HST housing 5 so as tocommunicate the inside space of HST housing 5 with the outside space ofHST housing 5. A bearing cap 8 is fitted into hole 5 b. A bearingbracket 9 is fitted into hole 5 c. Bearing cap 8 is formed at an outerend portion thereof with a flange 8 b. Flange 8 b is fitted onto one ofthe right and left outer side surfaces of HST housing 5 so as to coveran outer open end of hole 5 b. Similarly, bearing bracket 9 is formedwith a flange 9 b fitted onto the other of the right and left outer sidesurfaces of HST housing 5 so as to cover an outer open end of hole 5 c.

In this embodiment, right and left trunnion shafts of movable swashplate 6 are defined as a short trunnion shaft 6 a and long trunnionshaft 6 b. Bearing cap 8 and bearing bracket 9 are formed therein withrespective bearing holes 8 a and 9 a. Bearings 17 are provided inrespective bearing holes 8 a and 9 a so as to fit respective innerperipheral surfaces of bearing cap 8 and bearing bracket 9. A fluid seal18 is fitted in bearing hole 9 a of bearing bracket 9. Trunnion shaft 6a is inserted into bearing hole 8 a, and trunnion shaft 6 b into bearinghole 9 a, so that trunnion shafts 6 a and 6 b are journalled by bearingcap 8 and bearing bracket 9 via respective bearings 17. Therefore,flange 8 b of bearing cap 8 covers a tip of short trunnion shaft 6 a sothat the tip of trunnion shaft 6 a does not project outward from HSThousing 5. On the other hand, lateral bearing hole 9 a penetratesbearing bracket 9 so that an outer end of bearing hole 9 a is open atflange 9 b of bearing bracket 9. Therefore, a tip portion of longtrunnion shaft 6 b projects outward from flange 9 b of bearing bracket9, i.e., the outer end of bearing hole 9 a, thereby projecting outwardfrom HST housing 5. Fluid seal 18 prevents lubricating fluid fromleaking from bearing 17 in bearing bracket 9 to the outside of HSThousing 5. Further, a right or left side wall (in this embodiment, aright side wall) of vehicle body frame 11 having outer side surface 11 c(referred to in the first embodiment) is formed therethrough with a hole11 d, and the tip portion of trunnion shaft 6 b projects outward fromouter side surface 11 c of vehicle frame casing 11 via hole 11 d.

Bearing bracket 9 is formed with a foot portion 9 c projecting laterallyoutward from flange 9 b. Foot portion 9 c may be extended along an outerperipheral edge when viewed in the axial direction of trunnion shaft 6b. Alternatively, a plurality of foot portions 9 c may be aligned alongthe outer peripheral edge of flange 9 b. Foot portion 9 c projectsoutward from outer side surface 11 c of vehicle frame casing 11 via hole11 d. In this way, the tip portion of trunnion shaft 6 b and footportion 9 c of bearing bracket 9 are extended via hole 11 d into thespace below step 14 outside of vehicle frame casing 11. In thisembodiment, a servo unit 30, instead of servo set 40 of the firstembodiment, is disposed below step 14 and is attached to vehicle framecasing 11 by use of the tip portion of trunnion shaft 6 b and footportion 9 c of bearing bracket 9 projecting outward from vehicle framecasing 11.

Due to the joint of engine 10 to front open end 11 a of vehicle framecasing 11, fore-and-aft horizontal engine output shaft 10 a providedwith flywheel 10 b at a rear end portion thereof is extended rearwardinto vehicle frame casing 11 via front open end 11 a. Pump shaft 2 cserving as the input shaft of HST 1 is extended further forward fromgear casing 21 and is connected to flywheel 10 b via a damper 10 c.Similar to the first embodiment, pump shaft 2 c is drivingly connectedvia gears 22 and 23 in gear casing 21 to charge pump 20 in pump housing25 extended forward from gear casing 21.

Referring to 17, the HST hydraulic fluid supply system for supplying HST1 with hydraulic fluid delivered from charge pump 20 is configuredsimilar to that of the first embodiment as shown in FIG. 13. Moreover,in the second embodiment, referring to FIG. 14, a fluid hole 72 servesas fluid passage L2 for supplying fluid delivered from charge pump 20 toport P2. Fluid hole 72 is extended from the outside of axle casing 12into the inside of axle casing 12 through a hole 12 a bored in a wall ofaxle casing 12 so as to be joined to the rear open end of charge fluidpassage 4 a at the rear end surface of center section 4. Alternatively,fluid passage L2 in the second embodiment may be configured in anotherway. In this regard, charge fluid passage 4 a in center section 4 maynot be open to the inside of axle casing 12 as shown in FIG. 14.

Referring to FIG. 17, a general configuration of servo unit 30 and ahydraulic fluid supply system for supplying servo unit 30 with hydraulicfluid delivered from charge pump 20 will be described. Servo unit 30includes a servo housing 31 formed therein with a hydraulic cylinder132, similar to hydraulic cylinder 32 of servo unit 60 in the firstembodiment. Piston 133 and a spring 133 a for biasing piston 133 towarda neutral position of piston 133 are disposed in hydraulic cylinder 132formed in servo housing 31. Piston 133 divides the inside space of servohousing 31 serving as hydraulic cylinder 132 into front and rear fluidchambers 132 a and 132 b. In this regard, FIG. 17 (and later-discussedFIG. 24) illustrates spring 133 a as being different in structure fromspring 133 a shown in FIG. 16. However, the only purpose of illustrationof spring 133 a in the hydraulic circuit diagram of FIG. 17 (and FIG.24) is to indicate the function of spring 133 a to bias piston 133toward the neutral position. Therefore, spring 133 a may have anyconfiguration, such as shown in FIG. 16, only if spring 133 a has therequired function.

Referring to FIGS. 15 and 16, a concrete configuration of servo unit 30will be described while it is generally configured as mentioned abovewith reference to FIG. 17. Servo unit 30 is disposed just under step 14as mentioned above, so that step 14 is disposed immediately above a topsurface of servo unit 30. Therefore, proportional pressure controlvalves 35 and 36 are juxtaposed front and rear, and are disposed in alower portion 31L of servo housing 31 (hereinafter referred to as “lowerhousing portion 31L”). Proportional solenoids 35 a and 36 a ofrespective proportional pressure control valves 35 and 36 projectdownward from a bottom surface of servo housing 31. On the other hand,fore-and-aft horizontal hydraulic cylinder 132 is formed in upperportion 31U (hereinafter referred to as “upper housing portion 31U”).

Proportional pressure control valves 35 and 36 provided in lower housingportion 31L are provided at upper portions thereof with respectivesuction ports 35 b and 36 b, and at lower portions thereof withrespective drain ports 35 c and 36 c. A fore-and-aft horizontal suctionfluid hole 31 b is formed in lower housing portion 31L so as to fluidlyconnect suction ports 35 b and 36 b to each other via suction fluid hole31 b. A laterally horizontal suction fluid hole 31 a is formed in lowerhousing portion 31L so as to extend from a fore-and-aft intermediateportion of suction fluid hole 31 b in the direction laterally oppositeouter side surface 11 c of vehicle body frame 11 and HST 1. Hereinafter,in the description of servo unit 30 and later-discussed servo units 30A,30B, 30C, 30D and 30E as modifications of servo unit 30, this sidelaterally opposite vehicle frame casing 11 and HST 1 is defined as a“distal” side, while another side toward vehicle frame casing 11 and HST1 is defined as a “proximal” side. Suction hole 31 a has an outer endopen at a right or left “distal” side surface 31 j of servo housing 31(hereinafter referred to as “distal housing side surface 31 j”). Thisopen end of suction fluid hole 31 a serves as inlet port P3 shown inFIG. 17 so as to be joined to a fluid pipe or the like serving as fluidpassage L3.

A fore-and-aft horizontal drain fluid hole 31 c is formed in lowerhousing portion 31L below suction fluid hole 31 b so as to fluidlyconnect drain ports 35 c and 36 c to each other via drain fluid hole 31d. A laterally horizontal drain fluid hole 31 d is formed in lowerhousing portion 31L so as to extend distally from a fore-and-aftintermediate portion of drain fluid hole 31 c. Drain fluid hole 31 d hasan outer end open at distal housing side surface 31 j. This open end ofdrain fluid hole 31 d serves as outlet port P4 shown in FIG. 17 so as tobe joined to a fluid pipe or the like serving as fluid passage L4.

Servo housing 31 has a right or left (in this embodiment, left) proximalside surface 31 i (hereinafter referred to as “proximal housing sidesurface 31 i”) facing vehicle frame casing 11 and HST 1. Proximalhousing side surface 31 i is formed with a notch 31 g (see FIG. 15) overupper and lower housing portions 31U and 31L. An arm 34 is disposed innotch 31 g so as to serve as a connection member for connecting piston133 to trunnion shaft 6 b of movable swash plate 6 of HST 1. Further, ahole 31 h is formed in upper housing portion 31U so as to connect anupper portion of notch 31 g in upper housing portion 31U to hydrauliccylinder 132. An engagement portion 34 a projects from a top portion ofarm 34 disposed in the upper portion of notch 31 g so as to engage withpiston 133 in hydraulic cylinder 132 via hole 31 h. In this regard, hole31 h is fluidly connected to neither fluid chamber 132 a nor 132 bregardless of the slide direction or degree of piston 133. In otherwords, piston 133 keeps the fluidal tightness of fluid chambers 132 aand 132 b from notch 31 g.

In a lower portion of notch 31 g, arm 34 is formed at a lower portionthereof with an trunnion boss 34 b and a bolt boss 34 d joined coaxiallyto each other. A later-discussed cover member 37 journals trunnion boss34 b via a bearing, i.e., a later-discussed fluid seal 39. Trunnion boss34 b project proximally horizontally from cover member 37. A taperedrecess 34 c is formed in an end portion of trunnion boss 34 b projectingproximally from cover member 37. A tapered tip portion of trunnion shaft6 b is inserted into recess 34 c.

Distally horizontal bolt boss 34 d is extended laterally oppositetrunnion boss 34 b. A hole 31 r is bored through a boundary portion ofservo housing 31 between upper and lower housing portions 31U and 31L(i.e., below hydraulic cylinder 132 and above proportional pressurecontrol valves 35 and 36) so as to extend from notch 31 g on theproximal side of servo housing 31 to the distal end of the boundaryportion of servo housing 31. Bolt boss 34 d is passed through hole 31 rrotatably relative to servo housing 31 so as to project at an outer endthereof distally outward from distal housing side surface 31 j. Boltboss 34 d and trunnion boss 34 b are bored through by an axial bolt hole34 e from the distal outer end of bolt boss 34 d to recess 34 c intrunnion boss 34 b.

A cover member 37 is fixed to proximal housing side surface 31 i ofservo housing 31 so as to cover notch 31 g. More specifically, covermember 37 is fastened to servo housing 31 by at least one bolt 38. Ahole 37 a is provided in cover member 37. Trunnion boss 34 b of arm 34is passed through hole 37 a. A fluid seal 39 is provided in hole 37 a soas to keep the fluidal tightness of trunnion boss 34 b from cover member37. Bot boss 34 d is extended on the rotary axis of trunnion boss 34 b,and is passed through servo housing 31. In this way, arm 34 isjournalled at the lower portion thereof formed with laterally extendedtrunnion boss 34 b and bolt boss 34 d by cover member 37 and servohousing 31. Cover member 37 is formed with a foot portion 37 b extendedproximally therefrom so as to correspond to foot portion 9 c of bearingbracket 9.

Servo unit 30 is a unit including arm 34 and cover member 37. The tipportion of trunnion shaft 6 b projecting outward from vehicle framecasing 11 via hole 11 d and foot portion 9 c of bearing bracket 9, asmentioned above, are utilized to attach servo unit 30 to HST 1. In theseries of works for attaching servo unit 30 to HST 1, first, the taperedtip portion of trunnion shaft 6 b is fitted into tapered recess 34 cformed on trunnion boss 34 b of arm 34 projecting outward from covermember 37. The taper shapes of recess 34 c and the tip portion oftrunnion shaft 6 b become narrower as they go in the distal direction.Therefore, after the tip portion of trunnion shaft 6 b enters a littleinto recess 34 c, entire servo unit 30 is further pressed proximally.Finally, the tapered tip portion of trunnion shaft 6 b is completelyfitted into tapered recess 34 c, and servo unit 30 comes to be able tobe pressed no further proximally. Then, a bolt 50 is inserted into bolthole 34 e from the distal open end of bolt boss 34 d of arm 34projecting distally from servo housing 31. In this regard, a tapped hole6 c is axially formed in trunnion shaft 6 b and is open at the tip endsurface of trunnion shaft 6 b, and a threaded portion 50 a of bolt 50projecting proximally from bolt hole 34 e into recess 34 c is screwedinto tapped hole 6 c, thereby fixing arm 34 to trunnion shaft 6 b.

Further, a proximal side surface of foot portion 37 b of cover member 37contacts a distal side surface of foot portion 9 c of bearing bracket 9projecting from vehicle frame casing 11 via hole 11 d. Foot portion 37 bis fastened to foot portion 9 c via at least one bolt 51. In this way,cover member 37 is fixed to bearing bracket 9 so that servo housing 31outside of vehicle frame casing 11 is fixed to HST housing 5 inside ofvehicle frame casing 11, thereby completing the attachment of servo unit30 to HST 1.

Referring to FIG. 15, step 14 disposed above servo unit 30 is asubstantially horizontal plate that does not cover servo unit 30 at thedistal side of servo unit 30. In other words, servo unit 30 is open atthe distal portion thereof, so that an operator can easily handle servounit 30 to bring servo unit 30 to the attachment position below step 14,and then, the operator can easily perform the above-mentioned series ofworks for attaching servo unit 30 to HST 1, i.e., the location of servounit 30 relative to trunnion shaft 6 b by inserting the tip portion oftrunnion shaft 6 b into recess 34 c, the proximal pressing of servo unit30 for engaging arm 34 to trunnion shaft 6 b, and the screwing of bolts50 and 51. On the other hand, when servo unit 30 has to be detached fromHST 1 for the purpose of maintenance or the like, an operator alsobenefits facility in the series works, i.e., the access to servo unit 30below step 14, the releasing of bolts 50 and 51, and the distalwithdrawing of servo unit 30. Therefore, servo unit 30 needs neither itsown disassembling nor its own reassembling to be attached or detached toand from HST 1.

After the attachment of servo unit 30 to HST 1 is completed,proportional pressure control valves 35 and 36 are controlled to slidepiston 133 in hydraulic cylinder 132 in the fore-and-aft direction. Aspiston 133 slides, engagement portion 34 a of arm 34 rotates in thefore-and-aft direction centered on the axis of bolt 50. Accordingly,trunnion boss 34 b and bolt boss 34 d of arm 34 also rotate centered onthe axis of bolt 50. Therefore, trunnion shaft 6 b fixed to arm 34 viabolt 50 rotates integrally with trunnion boss 34 b centered on the axisof bolt 50, thereby tilting movable swash plate 6.

In this regard, the upper portion of arm 34 including engagement portion34 a moves vertically while it rotates in the fore-and-aft directionfollowing the fore-and-aft movement of piston 133 along its fore-and-afthorizontal axis. Notch 31 g and hole 31 h are formed so as to allow thisvertical movement of the upper portion of arm 34. Further, due to therotatability of bolt boss 34 d in through hole 31 r of servo housing 31relative to servo housing 31, the rotatability of arm 34 including boltboss 34 d centered on the axis of bolt 50 is ensured.

Referring to FIGS. 18 to 23, servo units 30A, 30B, 30C, 30D and 30Eserving as modifications of servo unit 30 according to the secondembodiment will be described. However, description of the componentelements designated by the same reference numerals as those used fordescription of the embodiment shown in FIGS. 14 to 17 will be omittedexcept for some special cases, on the assumption that they are identicalor similar to those designated by the same reference numerals in theembodiment of FIGS. 14 to 17.

Servo unit 30A will be described with reference to FIG. 18. Servo unit30A has a servo housing 31A that is similar to servo housing 31 of servounit 30 except that a lower portion of notch 31 g formed in servohousing 31A is expanded further downward so as to serve as a gallery 31m, and drain fluid hole 31 d extended distally from drain fluid hole 31c is not formed but a drain fluid hole 31 k is formed in servo housing31A so as to extend proximally from drain fluid hole 31 c and so as tobe open at proximal housing side surface 31 i to gallery 31 m.

Servo unit 30A also includes a cover member 37A that is similar to covermember 37 of servo unit 30 except that cover member 37A is not formedwith foot portion 37 b but is formed with a circular mount boss 37 csurrounding the entire outer peripheral surface of trunnion boss 34 b ofarm 34. Correspondingly, HST 1 is provided with a bearing bracket 9Athat is similar to bearing bracket 9 except that bearing bracket 9A isnot formed with foot portion 9 c but is formed with a circular mountboss 9 d surrounding the entire outer peripheral surface of trunnionshaft 6 b and projecting distally from flange 9 b. Mount boss 37 c andmount boss 9 d are joined to each other (i.e., fastened to each other bybolt 51) so as to form a gallery 52 that is a closed circular spacesurrounding trunnion shaft 6 b and trunnion boss 34 b.

Cover member 37A of servo unit 30A is formed so as to have a gap 37 bbetween cover member 37A and trunnion boss 34 b. Gap 37 d fluidlyconnects gallery 31 m in servo housing 31A to gallery 42 in mount bosses9 d and 37 c. Bearing bracket 9A adapted to have servo unit 30A has nofluid seal like fluid seal 18 in the outer side portion of bearing hole9 a therein.

Due to the above-mentioned structure, servo unit 30A can drain fluidfrom drain fluid hole 31 k in servo housing 31A to gallery 31 m in adirection designated by an arrow in FIG. 18. The drained fluid ingallery 31 m enters gallery 52 via gap 37 d, and is supplied fromgallery 52 to bearing 17 on trunnion shaft 6 b as lubricating fluid forbearing 17 and trunnion shaft 6 b. The fluid is further introduced intoHST housing 5 so as to lubricate the various parts of HST 1 in HSThousing 5. Referring to the hydraulic circuit diagram of FIG. 17, gap 37d serves as outlet port P4, notch 9 e serves as inlet port P5, andgallery 52 serves as fluid passage L4.

Servo unit 30B will be described with reference to FIG. 19. Similar toservo units 30 and 30A, servo unit 30B is fixed to trunnion shaft 6 b ofmovable swash plate 6 of HST 1 via hole 11 d of vehicle frame casing 11.However, servo housing 31 of servo unit 30B is not fixed to bearingbracket 9 but is fixed to vehicle frame casing 11, in comparison witheach of servo housings 31 and 31A of servo units 30 and 30A that isfixed to bearing bracket 9 of HST 1.

Servo unit 30B employs cover member 37A including circularly cylindricalmount boss 37 c, similar to servo unit 30A. Vehicle frame casing 11 isformed with a mount portion 1 e around hole 11 d. The proximal endsurface of mount boss 37 c contacts an end surface of mount portion 11e, and is fastened to mount portion 11 e by bolt 51, thereby fixingcover member 37A to vehicle frame casing 11, and thereby fixing servohousing 31 to vehicle fame casing 11.

In correspondence to servo unit 30B, HST 1 is provided with a bearingbracket 9B for journaling trunnion shaft 6 b. Since bearing bracket 9Bdoes not need to have a portion like foot portion 9 c or mount boss 9 dfor its joint to cover member 37 or 37A, bearing bracket 9B is formed atan outer end portion with a flange 9 b, similar to bearing cap 8.Therefore, while bearing bracket 9B does not project distally outwardfrom vehicle frame casing 11, trunnion shaft 6 b projects outward fromflange 9 b of bearing bracket 9B and further projects outward fromvehicle frame casing 11 via hole 11 d. In this way, bearing bracket 9Bis simplified in shape so as to reduce costs for its production.

Since cover member 37A of servo unit 30B is not joined to bearingbracket 9B, bearing 17 in bearing bracket 9B cannot be supplied withlubricating fluid from servo unit 30B via the fluid passage formed incover member 37A and bearing bracket 9B joined to each other. This isbecause servo unit 30B employs servo housing 30 identical to servohousing 30 of servo unit 30. In this regard, servo housing 31 is formedwith drain fluid hole 31 d open at distal housing side surface 31 j, andwith notch 31 g instead of downwardly expanded gallery 31 m. Further,cover member 37A of servo unit 30B is provided with no gap like gap 37 daround fluid seal 39, thereby ensuring its fluidal tightness.

Servo unit 30C will be described with reference to FIG. 20. Servo unit30C includes arm 34 fixed to trunnion shaft 6 b in the above-mentionedway. On the other hand, servo unit 30C includes a servo housing 31Cfixed to step 14. In this regard, a cover plate 53 is extendedvertically downward from step 14 at the distal side of servo unit 30C.Servo housing 31C is formed with a bolt boss 31 n that is extendeddistally so as to contact cover plate 53 at a distal side surfacethereof. A bolt 54 is screwed into bolt boss 31 n via cover plate 53 soas to fasten servo housing 31C to cover plate 53.

Servo unit 30C includes a cover member 37C. Cover member 37 is a simpleflat plate-shaped member that is not formed with a portion like footportion 37 b or mount boss 37 c. Therefore, simple bearing bracket 9Bhaving no portion to which cover member 37 or 37A is attached isprovided to journal trunnion shaft 6 b. Such simple and economic bearingbracket 9B and cover member 37C are used to constitute HST 1 with servounit 30C.

Cover plate 53 is provided with a hole 53 a through which bolt boss 34 dof arm 34 projects at an outer end portion thereof outward so that bolt50 can easily be inserted or withdrawn into and from bolt hole 34 e inbolt boss 34 d outside of cover plate 53.

Servo unit 30D will be described with reference to FIG. 21.Correspondingly, a step 55, instead of step 14, is fixed to outer sidesurface 11 c of vehicle frame casing 11 at a position lower than step14. Step 55 is stepped to have an upper step plate 55 a, a lower stepplate 55 b, and a vertical plate portion 55 c between upper and lowerstep plates 55 a and 55 b. Lower step plate 55 b of step 55 is lowerthan an upper end of hole 11 d in comparison with step 14 considerablyhigher than the upper end of hole 11 d. If the axis of trunnion shaft 6b were located between upper housing portion 31U of servo housing 31 or31A and proportional pressure control valves 35 and 36 in lower housingportion 31L, similar to that for servo units 30, 30A, 30B and 30C, upperhousing portion 31U above the axis of trunnion shaft 6 b would haveinterfered with lower step plate 55 b of step 55.

Therefore, servo unit 30D disposed below step 55 includes a servohousing 31D. An upper portion of upper housing portion 31U of servohousing 31D above hydraulic cylinder 132 is formed with an upwardextended portion 31Ua. Upper extended portion 31Ua of servo housing 31Dis formed with through hole 31 r through which bolt boss 34 d of arm 34is passed. Further, servo housing 31D is formed with an upward extendedportion 31Ub that is extended upward vertically from upward extendedportion 31Ua and along proximal housing side surface 31 i. Therefore,notch 31 g along proximal housing side surface 31 i is formed overupward extended portions 31Ua and 31Ub and upper housing portion 31U.Through hole 31 r is extended distally from an upper portion of notch 31g so as to be disposed above hydraulic cylinder 132.

In this way, upper housing portion 31U of servo housing 31D formed withhydraulic cylinder 32 is disposed below the axis of trunnion shaft 6 b,thereby enabling step 55 to be disposed at the position immediatelyabove upward extended portion 31Ua of servo housing 31D passing boltboss 33 d therethrough, i.e., the position of the axis of trunnion shaft6 b. The step-shape of step 55 corresponds to a step-shape formed byupper extended portions 31Ua and 31Ub of servo housing 31D. In thisregard, vertical plate portion 55 c faces a distal vertical side surfaceof upward extended portion 31Ub, upper step 45 a is disposed immediatelyabove the top of upward extended portion 31Ub, and lower step 45 bimmediately above the top of upward extended portion 31Ua.

Arm 34 adapted to servo unit 30D is arranged so as to have trunnion boss34 b and bolt boss 34 b at an upper portion thereof, and so as to haveengagement portion 34 a for engagement with piston 133 at a lowerportion thereof. In other words, arm 34 of servo unit 30D corresponds toupside-down reversed arm 34 of each of servo units 30, 30A, 30B and 30C.In this regard, servo unit 30D employs cover member 37 formed with footportion 37 b to journal arm 34, and HST 1 employs bearing bracket 9formed with foot portion 9 c to be joined to foot portion 37 b of covermember 37. However, in correspondence to the above-mentioned reversearrangement of arm 34, cover member 37 of servo unit 30D and bearingbracket 9 used for servo unit 30D correspond to upside-down reversedcover member 37 of servo unit 30 and upside-down reversed bearingbracket 9 for servo unit 30.

Alternatively, servo unit 30D may be provided with cover member 37Ahaving mount boss 37 c, and bearing boss 9A having mount boss 9 d may beprovided for servo unit 30D. However, if fluid drained from proportionalpressure control valves 35 and 36 has to be led from servo unit 30D tobearing 17 in bearing bracket 9A, horizontal fluid hole 31 k should bereplaced with another appropriate fluid guide structure to guide fluidfrom drain fluid hole 31 c to notch 31 g, because if servo housing 31Dis adapted to servo unit 30D, drain fluid hole 31 c in fluid housing 31Dbecomes considerably lower than notch 31 g.

Servo unit 30E will be described with reference to FIGS. 22 and 23. Incorrespondence to servo unit 30E, a step 56 is fixed to vehicle framecasing 11 at such a low position that is lower than step 55. Step 56 isstepped to have an upper step plate 56 a, a lower step plate 56 b and avertical plate portion 56 c between upper and lower step plates 56 a and56 b, similar to step 55. In comparison with step 55 having lower stepplate 55 b that is lower than the upper end of hole 11 d and higher thanthe axis of trunnion shaft 6 b, step 56 has upper step plate 56 a higherthan the axis of trunnion shaft 6 b, and has lower step plate 56 b lowerthan the axis of trunnion shaft 6 b. If servo unit 30D as shown in FIG.21 were disposed below step 56, bolt boss 34 d of arm 34, upwardextended portion 31Ua of servo housing 31D incorporating bolt boss 34 d,and upper housing portion 31U of servo unit 30D would interfere withlower step plate 56 b.

In correspondence to the low position of lower step plate 56 b of step56, servo unit 30E includes a servo housing 31E that is entirely lowerthan the axis of trunnion shaft 6 b. Therefore, a connection memberconnecting piston 133 to trunnion shaft 6 b does not need to have alaterally long portion, like bolt boss 34 d, through a servo housing.

On the other hand, since the lowering of servo housing 31E meanslowering of hydraulic cylinder 132, the connection member connectingpiston 133 to trunnion shaft 6 b must be vertically long. If theconnection member were an arm, like arm 34, rotatably centered on theaxis of trunnion shaft 6 b, it would be difficult for the connectionmember to have a portion, like engagement portion 34 a of arm 34,engaging with piston 133, because such a portion would greatly movevertically and in the fore-and-aft direction during the rotation of theconnection member arm centered on the axis of trunnion shaft 6 b.Further, it might be difficult for piston 133 to ensure the fluidictightness of fluid chambers 132 a and 132 b because hole 31 h would haveto be expanded to allow such a great movement of the portion of theconnection member engaging with piston 133.

Therefore, upper and lower sector gears 96 and 97 meshing with eachother serve as the connection member for connecting piston 133 of servounit 30E to trunnion shaft 6 b. Upper sector gear 96 is formed with atrunnion boss 96 a having a tapered recess 96 b, similar to trunnionboss 34 b of arm 34. Upper sector gear 96 is bored through between adistal side surface thereof and recess 96 b by a bolt hole 96 c. A bolt95 formed with a threaded shaft portion 96 a is inserted into bolt hole96 c from the distal side surface of sector gear 96, so that threadedshaft portion 96 a is screwed into tapped hole 6 c in trunnion shaft 6 bfitted in recess 96 b, thereby fixing sector gear 96 to trunnion shaft 6b. In this way, sector gear 96 and trunnion shaft 6 b are rotatablyintegral with each other and centered on the axis of trunnion shaft 6 band the axis of bolt 95 coaxial to trunnion shaft 6 b.

Gear teeth 96 d formed on a lower edge of upper sector gear 96 mesh withgear teeth 97 b formed on an upper edge of lower sector gear 97 disposedbelow upper sector gear 96. Lower sector gear 97 is formed at a bottomend thereof with an engagement portion 97 a similar to engagementportion 34 a of arm 34. Engagement portion 97 a engages with piston 133in hydraulic cylinder 132. Lower sector gear 97 is provided with a pivotshaft 98 at a vertical middle portion thereof between engagement portion97 a and gear teeth 97 b. When piston 133 engaging with engagementportion 97 a slides in the fore-and-aft direction, sector gear 97rotates centered on pivot shaft 98 so as to follow piston 133, therebyrotating sector gear 96 and movable swash plate 6 having trunnion shaft6 b.

Servo housing 31E is configured so as to support lower sector gear 97.In this regard, notch 31 g of servo housing 31E formed along proximalhousing side surface 31 i accommodates a lower portion of sector gear97. Servo housing 31 has an opening at an upper end of notch 31 g sothat an upper portion of sector gear 97 projects upward from servohousing 31E via the opening at the upper end of notch 31 g. Pivot shaft98 is disposed on the upper portion of sector gear 97 projecting upwardfrom servo housing 31E, and is disposed immediately above the opening atthe upper end of notch 31 g. Servo housing 31E is formed with an upwardextended portion 31Uc that is extended upward from upper housing portion31U formed therein with hydraulic cylinder 32 so as to support a distalend portion of pivot shaft 98. Therefore, servo housing 31E does nothave a hole like through hole 31 r, thereby reducing a gap betweenhydraulic cylinder 132 and proportional pressure control valves 35 and36, and thereby being minimized vertically.

To attach servo unit 30E, servo housing 31E is fixed to a bearingbracket 9A of HST 1. Incidentally, bearing bracket 9A includes acircularly cylindrical mount boss 9 d that has a sufficient strength forsupporting sector gears 96 and 97. Alternatively, bearing bracket 9including foot portion 9 c may be used if there is no problem in thestrength.

Servo unit 30E includes a bearing member 57 to be attached to bearingbracket 9A. A lower portion of bearing member 57 is joined to proximalhousing side surface 31 i of servo housing 31E so as to cover notch 31g. Bolt 38 (not shown) may be used to fasten bearing member 57 to servohousing 31 i. Bearing member 57 is vertically extended along proximalside surfaces of sector gears 96 and 97 so as to have the proximal endportion of pivot shaft 98 of sector gear 97 supported by the verticallymiddle portion of bearing member 57. Bearing member 57 is formed in anupper portion thereof with a bearing hole 57 a having fluid seal 39therein, similar to bearing hole 37 a of cover member 37. Trunnion boss96 a of sector gear 96 is passed through bearing hole 57 a and isjournalled by bearing member 57 via fluid seal 39.

Further, in correspondence to mount boss 9 d of bearing bracket 9A, theupper portion of bearing member 57 is formed with a circularlycylindrical mount boss 57 b, similar to mount boss 37 c of cover member37A. To attach servo unit 30E, mount boss 57 b contacts mount boss 9 bof bearing bracket 9A, and bolt 51 fastens mount boss 57 b to mount boss9 d of bearing bracket 9A so as to fix servo housing 31E to bearingbracket 9A.

Incidentally, in servo unit 30 or so on, arm 34 is entirely covered withservo housing 31 and/or cover member 37 except that only the outer endportion of bolt boss 34 d projects outward from servo housing 31. On theother hand, in servo unit 30E, bearing member 57 is disposed at theproximal side of sector gears 96 and 97, however, entire sector gear 96and the upper portion of sector gear 97 are extended upward from servohousing 31E so that their distal side surfaces may be exposed.Therefore, a cover 58 is extended from an upper portion of bearingmember 57 to a top of upward extended portion 31Uc of servo housing 31Eso as to cover the distal side surface of entire sector gear 96 and thedistal side surface of the upper portion of sector gear 97.

Incidentally, the head of bolt 95 at the distal side surface of sectorgear 96 is also covered with cover 58. Therefore, to enable an operatorto access the head of bolt 95, cover 58 may be detachable, or a wrenchhole may be provided in cover 58 at a position corresponding to the headof bolt 95 so that an adjustable wrench can be inserted from the outsideof cover 58 into the wrench hole so as to engage to the head of bolt 95.Further, cover 58 should be disposed along vertical plate portion 56 bof step 56. Therefore, another wrench hole may be provided in verticalplate portion 56 b of step 56 so that the adjustable wrench can beengages to the head of bolt 95 via the wrench holes in vertical plateportion 56 c of step 56 and cover 58.

All servo units 30, 30A, 30B, 30C, 30D and 30E employ the hydrauliccircuit structure shown in FIG. 17, i.e., rely on the assumption thateach of them is an assembly as combination of hydraulic cylinder 132incorporating piston 133 and proportional pressure control valves 35 and36 for controlling the hydraulic pressures in fluid chambers 132 a and132 b of hydraulic cylinder 132. In this regard, in FIG. 17, thereference numeral “30” is used to designate a representative servo unitfor all servo units 30, 30A, 30B, 30C, 30D and 30E. Hereinafter, thisrepresentative servo unit is referred to as “servo unit 30”.

An alternative servo unit 90 having a hydraulic circuit structure asshown in FIG. 24 will be described. Servo unit 90 includes a servohousing 91 formed therein with hydraulic cylinder 132, similar to servohousing 31. Servo housing 91 is provided with inlet port P3 forreceiving fluid from charge pump 20, and with outlet port P4 fordischarging fluid therefrom. Solenoid valves 92 and 93 are assembled inservo housing 91. Solenoid valve 92 is a proportional pressure controlvalve 92 that is expensive, while solenoid valve 93 is a simpledirectional control valve 93 that is economic.

In this regard, servo unit 30 includes two proportional solenoid valvesserving as proportional pressure control valves 35 and 36, so that oneis provided for forward traveling of the vehicle, and the other forbackward traveling of the vehicle. In other words, one is provided forproportionally controlling the hydraulic pressure of fluid in fluidchamber 132 a, and the other for proportionally controlling thehydraulic pressure of fluid in fluid chamber 132 b. Therefore, servounit 30 is expensive due to the two proportional solenoid valves. On thecontrary, servo unit 90 includes only one proportional solenoid valvethat is proportional pressure control valve 92 provided with aproportional solenoid 92 a. Directional control valve 93 is economicbecause whether its solenoid is excited or not simply depends on whethera reversing (i.e., selecting either the forward or backward travelingdirection) manipulator, such as a pedal or a lever, is set at a forwardtraveling position or a backward traveling position.

Proportional pressure control valve 92 includes three ports, i.e., asuction port 92 b, a drain port 92 c, a valve port 92 d. Directionalcontrol valve 93 includes four ports, i.e., a valve port 93 a, a drainport 93 b, a first fluid chamber connection port 93 c, and a secondfluid chamber connection port 93 d. Suction port 92 b of proportionalpressure control valve 92 is fluidly connected to inlet port P3 via afluid passage 91 a. Drain port 92 b of proportional pressure controlvalve 92 is fluidly connected to outlet port P4 via a fluid passage 91b. Valve port 92 d of proportional pressure control valve 92 is fluidlyconnected to valve port 93 a of directional control valve 93 via a fluidpassage 91 c. Drain port 93 b of directional control valve 93 is fluidlyconnected via a fluid passage 91 d to fluid passage 91 b extended fromdrain port 92 c of proportional pressure control valve 92 to outlet portP4. Fluid drained from proportional pressure control valve 92 and fluidfrom directional control valve 93 are joined to each other via fluidpassages 91 b and 91 d so as to be drained outward from servo housing 91via outlet port P4. First fluid connection port 93 c of directionalcontrol valve 93 is fluidly connected to fluid chamber 132 a via a fluidpassage 91 e. Second fluid connection port 93 d of directional controlvalve 93 is fluidly connected to fluid chamber 132 b via a fluid passage91 f.

Proportional pressure control valve 92 is vibratorily shifted betweensupply position S and drain position D due to the control of currentapplied to proportional solenoid 92 a. Proportional pressure controlvalve 92 set at supply position S fluidly connects valve port 92 d tosuction port 92 b so as to supply fluid from valve port 92 d to valveport 93 a of directional control valve 93 via fluid passage 91 c.Proportional pressure control valve 92 set at drain position D fluidlyconnects valve port 92 d to drain port 92 c so as to drain fluid fromvalve port 93 a of directional control valve 93 to fluid passage 91 bconnected to outlet port P4 via fluid passage 91 c. Due to the repeat ofvibratory shift of proportional pressure control valve 92 between supplypositon S and drain position D, a certain hydraulic pressure is given tovalve port 93 a.

Directional control valve 93 is shifted between a first position A and asecond position B by on-off switching of its solenoid depending on theshift of the reversing manipulator between the forward traveling positonand the backward traveling position. One of first and second positions Aand B corresponds to the forward traveling positon, and the othercorresponds to the backward traveling position. Hereinafter, descriptionwill be based on an assumption that first position A corresponds to theforward traveling position, and second position B corresponds to thebackward traveling position.

Directional control valve 93 set at first position A fluidly connectsvalve port 93 a to first fluid chamber connection port 93 c, and drainport 93 b to second fluid chamber connection port 93 d. Therefore, whendirectional control valve 93 is set at first position A, fluid chamber132 b is fluidly connected via directional control valve 93 to fluidpassage 91 d connected to drain port P4) bypassing proportional pressurecontrol valve 92, regardless of the set position of proportionalpressure control valve 92, and fluid chamber 132 a is fluidly connectedto valve port 92 d of proportional pressure control valve 92 viadirectional control valve 93.

During the setting of directional control valve 93 at first position A,when proportional pressure control valve 92 is set at supply position S,fluid chamber 132 a is fluidly connected to suction port 92 b ofproportional pressure control valve 92 constantly connected to inletport P3, so that fluid chamber 132 a is supplied with hydraulic fluidfrom inlet port P3 via proportional pressure control valve 92 so as tobe expanded to press piston 133 in the direction to fluid chamber 132 b.Due to the pressure of piston 133 by the hydraulic expansion of fluidchamber 132 a, fluid is naturally discharged from fluid chamber 132 b tooutlet port P4 via directional control valve 93 and fluid passage 91 d.On the contrary, during the setting of directional control valve 93 atfirst position A, when proportional pressure control valve 92 is set atdrain position D, fluid chamber 132 a is fluidly connected to drain port92 c of proportional pressure control valve 92 constantly connected tofluid passage 91 b, which is joined to fluid passage 91 d and isextended to outlet portion P4. Therefore, both fluid chambers 132 a and132 b are fluidly connected not to inlet port P3 but to each other andto outlet port P4, so that the fluid flow between fluid chambers 132 aand 132 b through valves 92 and 93 and fluid passages 91 b and 91 d isnaturally adjusted freely from the hydraulic pressure of fluid suppliedfrom inlet port P3 so as to locate piston 133 at a position where fluidchambers 132 a and 132 b are balanced in hydraulic pressure. Whiledirectional control valve 93 is set at first position A for forwardtraveling, finally, piston 133 reaches a target position closer to oneend of hydraulic cylinder 132 at fluid chamber 132 b side than the otherend of hydraulic cylinder 132 at fluid chamber 132 a side, so thatmovable swash plate 6 is located at a corresponding target tilt angle inthe tilt direction for forward traveling.

Directional control valve 93 set at first position B fluidly connectsvalve port 93 a to second fluid chamber connection port 93 d, and drainport 93 b to first fluid chamber connection port 93 c. Therefore, whendirectional control valve 93 is set at second position B, fluid chamber132 a is fluidly connected via directional control valve 93 to fluidpassage 91 d (connected to outlet port P4) bypassing proportionalpressure control valve 92, regardless of the set position ofproportional pressure control valve 92, and fluid chamber 132 b isfluidly connected to valve port 92 d of proportional pressure controlvalve 92 via directional control valve 93.

During the setting of directional control valve 93 at first position B,when proportional pressure control valve 92 is set at supply position S,fluid chamber 132 b is fluidly connected to suction port 92 b ofproportional pressure control valve 92 constantly connected to inletport P3, so that fluid chamber 132 b is supplied with hydraulic fluidfrom inlet port P3 via proportional pressure control valve 92 so as tobe expanded to press piston 133 in the direction to fluid chamber 132 a.Due to the pressure of piston 133 by the hydraulic expansion of fluidchamber 132 b, fluid is naturally discharged from fluid chamber 132 a tooutlet port P4 via directional control valve 93 and fluid passage 91 d.On the contrary, during the setting of directional control valve 93 atfirst position B, when proportional pressure control valve 92 is set atdrain position D, fluid chamber 132 b is fluidly connected to drain port92 c of proportional pressure control valve 92 constantly connected tofluid passage 91 b, which is joined to fluid passage 91 d and isextended to outlet portion P4. Therefore, both fluid chambers 132 a and132 b are fluidly connected not to inlet port P3 but to each other andto outlet port P4, so that the fluid flow between fluid chambers 132 aand 132 b through proportional pressure control valves 92 and 93 andfluid passages 91 b and 91 d is naturally adjusted so as to locatepiston 133 at a position where fluid chambers 132 a and 132 b arebalanced in hydraulic pressure. While directional control valve 93 isset at first position B for backward traveling, finally, piston 133reaches a target position closer to one end of hydraulic cylinder 132 atfluid chamber 132 a side than the other end of hydraulic cylinder 132 atfluid chamber 132 b side, so that movable swash plate 6 is located at acorresponding target tilt angle in the tilt direction for backwardtraveling.

The above-mentioned configuration of servo unit 90 shown in FIG. 24 mayadapted to any of servo unit 30 to 30E shown in FIGS. 15, 16, and 18 to23. In other words, servo unit 90 is provided with a connection meansfor connecting piston 133 to trunnion shaft 6 b. This connection meansmay be either an arm like arm 34 employed by each of servo units 30,30A, 30B, 30C and 30D or sector gears like sector gears 96 and 97employed by servo unit 30E.

If servo housing 91 is configured so as to be fixed to bearing bracket 9or 9A of HST 1, servo unit 90 may be provided with any one of covermembers 37, 37A and 37D and bearing member 57. If servo housing 91 isconfigured so as to be fixed to vehicle frame casing 11, servo unit 90may have the structure as shown in FIG. 19. If servo housing 91 isconfigured so as to be fixed to step 14, servo unit 90 may have thestructure as shown in FIG. 20. Servo housing 91 may have the structureof any one of servo housings 31, 31A, 31C, 31D and 31E depending onwhich portion of servo housing 91 should have outlet port P4, whichmember or portion should have servo housing 91 fixed thereto, or so on.

Next, still another embodiment of a servo set for controlling a movableswash plate of an HST will be described with reference to FIGS. 25 to35. Further, the members and portions designated by the referencenumerals used in the aforementioned description are identical or similarto the respective members and portions designated by the same referencenumerals and, therefore, will not be described redundantly.

It is an object of this embodiment to provide an actuator device that isreduced in size, thereby downsizing the HST equipped with the actuatordevice.

Similarly to those in the first embodiment described with reference toFIGS. 1 and 2, as illustrated in FIG. 33, axle casing 12 and vehicleframe casing 11 interiorly incorporating HST1 (not illustrated) areprovided, and servo set 200 is disposed on the right side portion ofvehicle frame casing 11 and axle casing 12.

First, with reference to FIGS. 25 to 33, the structure of servo set 200will be described. Servo set 200 described in the present embodimentincludes servo portion 210 which is a portion having the same functionsas those of the aforementioned servo unit 60, and neutral returningportion 220 which is a portion having the same functions as those of theaforementioned neutral returning unit 80, which are integrated with eachother.

Servo portion 210 is constituted by servo main body 211 (which is alsothe cylinder case), piston 212, piston rod 213, and proportionalpressure control valves 214 and 215, as illustrated in FIG. 29.

Servo main body 211 forms a middle portion of servo set 200 in theforward and rearward direction and is a case member corresponding to theintegration of hydraulic cylinder 32, valve block 64 and fluid ductblock 65 which have been described above. Servo main body 211 isprovided with cylinder 211 a, rod hole 221 b, valve holes 211 c and 211d, fluid passages 211 e and 211 f. Further, servo main body 211 also hasthe function (role) of a bracket for mounting servo set 200 to vehicleframe casing 11. As illustrated in FIG. 33, servo main body 211 isextended substantially in the forward and rearward direction along outerside surface 11 c on the right side of vehicle frame casing 11, withplate member 203 interposed therebetween. Servo main body 211 ispivotally supported, at its middle portion in the forward and rearwarddirection, on vehicle frame casing 11, through pivot shaft 202.

Cylinder 211 a is a vacant portion with a substantially cylindricalshape for slidably housing piston 212, which is formed in servo mainbody 211. Cylinder 211 a is partitioned, by piston 212, into fluidchamber 211 g on the front side and fluid chamber 211 h on the rearside.

Piston 212 is slidably provided within cylinder 211 a, and piston rod213 is extended forwardly and rearwardly therefrom on the axis center ofpiston 212. With this structure, if piston 212 is displaced in theforward and rearward direction within cylinder 211 a, piston rod 213 isdisplaced along with the displacement of piston 212.

Front half portion 213 a of piston rod 213, which is the front portionthereof with respect to piston 212, is extended up to the outside of thefront side of servo main body 211 through rod hole 211 b, which enablesincreasing and decreasing the length of the protrusion of front halfportion 213 a from servo main body 211 to the outside. Further, rearhalf portion 213 b of piston rod 213, which is the rear portion thereofwith respect to piston 212, is extended rearwardly toward neutralreturning portion 220, which enables increasing and decreasing thelength of the protrusion of rear half portion 213 b from servo main body211 toward neutral returning portion 220.

The portion of servo main body 211 which closes the front-end sideopening portion of cylinder 211 a forms a supporting base portion forproportional pressure control valves 214 and 215. Proportional pressurecontrol valves 214 and 215 are screwed into valve holes 211 c and 211 dwhich are formed in the supporting base portion.

Fluid passage 211 e is formed within the wall of servo main body 211, insuch a way as to cause cylinder 211 a and valve hole 211 c tocommunicate with each other. Fluid passage 211 e is connected to thefront end portion of cylinder 211 a, thereby communicating with fluidchamber 211 g. Fluid passage 211 f is formed within the wall of servomain body 211, in such a way as to cause cylinder 211 a and valve hole211 d to communicate with each other. Fluid passage 211 f is connectedto the rear end portion of cylinder 211 a, thereby communicating withfluid chamber 211 h.

Proportional pressure control valve 214 is adapted to supply ordischarge a pressurized fluid to or from fluid chamber 211 g formed onthe front side with respect to piston 212 within cylinder 211 a.Proportional pressure control valve 215 is adapted to supply ordischarge the pressurized fluid to or from fluid chamber 211 h formed onthe rear side with respect to piston 212 within cylinder 211 a.Proportional pressure control valves 214 and 215 are proportionalsolenoid valves provided with proportional solenoids 214 a and 215 a,respectively. Proportional pressure control valves 214 and 215 areelectrically connected, through wirings 230 and 230 and couplers 231 and231, to a control device, which is not illustrated, for controlling theoperations of respective proportional pressure control valves 214 and215.

Proportional pressure control valve 214 includes supply/discharge port214 b adapted to communicate with fluid chamber 211 g through fluidpassage 211 e. Proportional pressure control valve 215 includessupply/discharge port 215 b adapted to communicate with fluid chamber211 h through fluid passage 211 f. One of proportional pressure controlvalves 214 and 215 is designated as a proportional pressure controlvalve to be excited in determining the forward travelling speed, whilethe other is designated as a proportional pressure control valve to beexcited in determining the rearward travelling speed. Throughsupply/discharge port 214 b or 215 b in the proportional pressurecontrol valve being excited, the pressurized fluid is supplied out ofproportional pressure control valves 214 and 215 to the correspondingfluid chamber 211 g or 211 h via the corresponding fluid passage 211 eor 211 f.

Further, servo main body 211 includes inlet port P3 and outlet port P4.Inlet port P3 communicates with fluid supply port 214 c of proportionalpressure control valve 214 and with fluid supply port 215 c ofproportional pressure control valve 215, through fluid passage 216.Outlet port P4 communicates with fluid discharge port 214 d ofproportional pressure control valve 214 and with fluid discharge port215 d of proportional pressure control valve 215, through fluid passage217.

With this structure, proportional pressure control valve 214 is adaptedto move a spool (not illustrated), through excitation of proportionalsolenoid 214 a, for bringing fluid supply port 214 c into an “opened”state while bringing fluid discharge port 214 d into a “closed” state,thereby supplying the pressurized fluid supplied through inlet port P3to fluid chamber 211 g via fluid passage 211 e, through fluid supplyport 214 c and supply/discharge port 214 b. Further, in demagnetizationstates, fluid supply port 214 c is in a “closed” state while fluiddischarge port 214 d is in an “opened” state, so that the pressurizedfluid accumulated in fluid chamber 211 g is discharged from outlet portP4, through supply/discharge port 214 b and fluid discharge port 214 d,via fluid passage 211 e.

Further, proportional pressure control valve 215 is adapted to move aspool (not illustrated) through excitation of proportional solenoid 215a, for bringing fluid supply port 215 c into an “opened” state whilebringing fluid discharge port 215 d into a “closed” state, therebysupplying a hydraulic fluid supplied through inlet port P3 to fluidchamber 211 h via fluid passage 211 f, through fluid supply port 215 cand supply/discharge port 215 b. Further, in demagnetization states,fluid supply port 215 c is in a “closed” state while fluid dischargeport 215 d is in an “opened” state, so that the pressurized fluidaccumulated in fluid chamber 211 h is discharged from outlet port P4,through supply/discharge port 215 b and fluid discharge port 215 d, viafluid passage 211 f.

As illustrated in FIG. 26, the positional relationship between cylinder211 a and proportional pressure control valves 214 and 215 in servo set200 is such that the axis center of piston rod 213 and the axis centersof the displacements of the valve bodies in respective proportionalpressure control valves 214 and 215 are placed at positions forming atriangle shape, when viewed in the axial direction of cylinder 211 a.With this placement, it is possible to reduce the size of servo set 200in the radial direction (in the direction orthogonal to the axialdirection of piston rod 213).

Servo portion 210 includes thread portion 218 at the front end of fronthalf portion 213 a of piston rod 213, and connection member 219 with acap shape is screwed around this thread portion 218. Connection member219 is adapted such that the depth of screwing thereof around threadportion 218 can be adjusted. Connection member 219 includes threadportion 219 a protruded in the direction orthogonal to the axis line ofpiston rod 213 in the state of being screwed around thread portion 218.Further, the position of connection member 219 relative to threadportion 218 can be fixed by nut 219 b screwed around thread portion 218.

Servo portion 210 includes flat portion 213 d at the front end of fronthalf portion 213 a of piston rod 213. Flat portion 213 d can be madeengageable with a tool such as a wrench, in order that piston rod 213can be easily rotated about the axis center thereof.

Thread portion 219 a at the front end of piston rod 213 is pivotallycoupled to arm member 201 (which is the manipulation lever) secured totrunnion shaft 6 b, so that servo portion 210 is connected to movableswash plate 6 (not illustrated) in HST 1. Further, in servo portion 210,by rotating piston rod 213 about its axis center with a tool such as awrench using flat portion 213 d, in the state where thread portion 219 aat the front end of piston rod 213 is pivotally coupled to arm member201, it is possible to adjust the depth of screwing of connection member219 around thread portion 218, thereby easily setting the neutralposition of piston rod 213.

Namely, as illustrated in FIG. 29, in servo set 200, piston rod 213 isprovided with thread portion 219 a at the tip end portion of front halfportion 213 a, and piston rod 213 is secured, through screwing, to armmember 201 through thread portion 219 a. Piston rod 213 is also providedwith flat portion 213 d in at least a portion of the outer peripheralsurface of piston rod 213. With this structure, it is possible to easilyadjust the neutral position of servo set 200.

Further, servo portion 210 is secured, through pivot shaft 202, to platemember 203, which acts as a mounting part and is rotatably and pivotallysupported with respect to shaft hole 11 f, which is a concave portionformed in vehicle frame casing 11. Plate member 203 is provided withshaft hole 203 b adapted to allow pivot shaft 202 to be insertedtherethrough, and bolt holes 203 c and 203 d for use in securing servoportion 210, in plate portion 203 a for supporting servo portion 210.Plate member 203 is provided on the side portion of vehicle frame casing11, by inserting a bolt as pivot shaft 202 into shaft hole 203 b andshaft hole 11 f formed in vehicle frame casing 11 and, further, byscrewing nut 204 around pivot shaft 202. At this time, collar 205 isinterposed between pivot shaft 202 and shaft hole 203 b, and washer 206is interposed between vehicle frame casing 11 and plate portion 203 a,so that plate member 203 is smoothly rotated about pivot shaft 202.

Servo portion 210 is secured to plate member 203 by inserting respectivebolts 207 and 208 into through holes 211 m and 211 n formed in servomain body 211 and, further, by screwing respective bolts 207 and 208into bolt holes 203 c and 203 d in plate member 203. As plate member 203oscillates about pivot shaft 202, servo portion 210 can also oscillate.

Neutral returning portion 220 is constituted by cylindrical member 221,neutral biasing springs 222 and 223, spring retainers 224 and 225, nuts226 and 227, retaining ring 228, and cover 229.

Cylindrical member 221 is provided with boss portion 221 a with asubstantially-cylindrical shape which protrudes forwardly, and rod hole221 b penetrated through boss portion 221 a on its axis center. Bossportion 221 a is fitted into a rear-end opening in servo main body 211,so that the cylindrical member 221 is provided coaxially with cylinder211 a on the rear end portion of servo main body 211 in servo portion210. Cylindrical member 221 is extended along outer side surface 11 c onthe right side of vehicle frame casing 11.

Cylindrical member 221 is sealed at its front end by boss portion 221 aand is opened at its rear end. Rear half portion 213 b of piston rod 213is inserted through rod hole 221 b in the forward and rearwarddirection, in such a way as to penetrate through cylindrical member 221.Rear half portion 213 b of piston rod 213 can be extended rearwardlyfrom the rear end of cylindrical member 221. Piston rod 213 can slide inthe axial direction of rod hole 221 b (in the forward and rearwarddirection). When piston rod 213 slides forwardly, the rear end of rearhalf portion 213 b of piston rod 213 can protrude rearwardly from therear end of boss portion 221 a.

Within cylindrical member 221, a pair of front and rear spring retainers224 and 225 are fitted to rear half portion 213 b of piston rod 213, insuch a way as to be slidable in the axial direction with respect tocylindrical member 221. Further, between the pair of spring retainers224 and 225 within cylindrical member 221, two neutral biasing springs222 and 223 with a coil spring shape and with different spring diametersare interposed in such a way as to surround piston rod 213. Neutralbiasing spring 223 with a smaller spring diameter is arranged insideneutral biasing spring 222 with a larger spring diameter, coaxially withpiston rod 213. Further, retaining ring 228 is secured to the innerperipheral portion of the rear end portion of cylindrical member 221.The rear end surface of boss portion 221 a in cylindrical member 221restricts the forward sliding of front spring retainer 224, andretaining ring 228 restricts the rearward sliding of rear springretainer 225. Further, cover 229 is provided on the rear portion ofcylindrical member 221, in order to protect the rear portion of pistonrod 213 protruding rearwardly, during rearward sliding of piston 212.Cover 229 is made of a rubber with elasticity, and is secured tocylindrical member 221 by fastening band 229 a fitted to the outer sideof cover 229.

Namely, as illustrated in FIG. 29, plural neutral biasing springs 222and 223 which constitute neutral returning portion 220 in servo set 200are arranged in the axial direction, such that the coil axis centers ofrespective neutral biasing springs 222 and 223 and the axis center ofpiston rod 213 are substantially coincident with each other. With thisplacement, it is possible to reduce the size of servo set 200 in theradial direction (in the direction orthogonal to the axial direction ofpiston rod 213).

With regard to the arrangement of servo portion 210 and neutralreturning portion 220 in servo set 200, piston rod 213 and respectiveproportional pressure control valves 214 and 215 are placed such thatthe axial direction of piston rod 213 and the directions ofdisplacements of the valve bodies of respective proportional pressurecontrol valves 214 and 215 are parallel with each other. Further,respective proportional pressure control valves 214 and 215 are placedon the end portion of one side of servo main body 211 constituting servoportion 210 and, also, respective neutral biasing springs 222 and 223are placed on the end portion of the other side (the opposite side tothe side provided with respective proportional pressure control valves214 and 215) of servo main body 211. With this placement, it is possibleto reduce the size of servo set 200 in the radial direction (in thedirection orthogonal to the axial direction of piston rod 213).

In neutral returning portion 220, neutral biasing springs 222 and 223are in an initial compressed state, and the spring forces of neutralbiasing springs 222 and 223 in the forward and rearward and outwarddirections bring both front and rear spring retainers 224 and 225 intopress contact with the rear end surface of boss portion 221 a and withretaining ring 228, respectively. Further, piston rod 213 is providedwith stepped portion 213 c halfway through its rear half portion 213 bin the axial direction, and stepped portion 213 c is in contact withfront spring retainer 224 at its front side, while front spring retainer224 is in contact with the rear end surface of boss portion 221 a. Nut226 is screwed around rear half portion 213 b of piston rod 213, and isin contact with spring retainer 224 at its rear side. Thus, steppedportion 213 c and nut 226 sandwich front spring retainer 224therebetween, so that spring retainer 224 is secured with respect topiston rod 213. Further, nut 227 is screwed around rear half portion 213b of piston rod 213, and nut 227 is in contact with rear spring retainer225 at its rear side. Thus, nut 227 restricts the position to which rearspring retainer 225 can slide rearwardly. Further, nut 227 isconstituted by a double nut.

Neutral position of the piston rod 213 is set to the position in theinitial state determined by neutral biasing springs 222 and 223 asillustrated in FIG. 29, in the direction of sliding of piston rod 213,when proportional solenoids 214 a and 215 a are both demagnetized. Thestructure for connecting movable swash plate 6 and piston rod 213 toeach other is set such that movable swash plate 6 is at a neutralposition when piston rod 213 is at the neutral position.

If the hydraulic pressure in fluid chamber 211 g exceeds the biasingforce of neutral biasing springs 222 and 223 as a result of excitationof proportional solenoid 214 a, the tip end of piston rod 213 slidesleftwardly in the paper plane of FIG. 29 from the neutral positionthereof, which causes trunnion shaft 6 b to rotate in theforward-travelling direction, through front half portion 213 a of pistonrod 213, thread portion 218, connection member 219 and arm member 201.At this time, servo main body 211 of servo portion 210 oscillates aboutpivot shaft 202 to absorb the difference between the circumferentialmovement of arm member 201 as the rotary member and the linear movementof piston rod 213 as a telescopic member.

Further, during the leftward sliding of piston rod 213, rear springretainer 225 is kept in contact with rear retaining ring 228, whilefront spring retainer 224 is pushed to move rearwardly by steppedportion 213 c of piston rod 213. Thus, neutral biasing springs 222 and223 are further compressed from the initial compressed state to apply aforward biasing force to piston rod 213. This biasing force forms aforward biasing force for restoring piston rod 213 (and, therefore,piston 212 and movable swash plate 6) to the neutral position.

On the other hand, if the hydraulic pressure in fluid chamber 211 hexceeds the biasing force of neutral biasing springs 222 and 223 as aresult of excitation of aforementioned proportional solenoid 215 a, thetip end of piston rod 213 slides rightwardly in the paper plane of FIG.29 from the neutral position thereof, which causes trunnion shaft 6 b torotate in the rearward-travelling direction, through front half portion213 a of piston rod 213, thread portion 218, connection member 219 andarm member 201. At this time, servo main body 211 of servo portion 210oscillates about pivot shaft 202 similarly to the aforementioneddescription, to absorb the difference between the rotation of arm member201 and the linear movement of piston rod 213.

During the rightward sliding of piston rod 213, front spring retainer224 is kept in contact with the rear end surface of boss portion 221 a,while rear spring retainer 225 is pushed to move forwardly by nut 227provided on the rear end portion of piston rod 213. Thus, neutralbiasing springs 222 and 223 are further compressed from the initialcompressed state to apply, to piston rod 213, a rearward biasing forcefor restoring piston rod 213 (and, therefore, piston 212 and movableswash plate 6) to the neutral position.

As described above, the neutral position of piston rod 213 defines theneutral position of piston 212 (in the state where proportionalsolenoids 214 a and 215 a are demagnetized) and, also, defines theneutral position of movable swash plate 6 through arm member 201.Namely, neutral returning portion 220 restores piston rod 213 to theneutral position through the biasing force of neutral biasing springs222 and 223, thereby restoring piston 212 of servo portion 210 to theneutral position thereof.

Further, as illustrated in FIG. 35, in servo set 200, servo portion 210constituting servo main body 211 is mounted to plate member 203, suchthat the longitudinal axis line α of piston rod 213 and the axis line βof pivot shaft 202 (the rotational center of plate member 203) areoverlapped with each other, when viewed in a cross section. With thisstructure, it is possible to efficiently transmit the forward andrearward strokes of piston rod 213 to arm member 201, while furthersmoothening the oscillation of servo set 200, during speed-changingmanipulations.

As illustrated in FIG. 35, pivot shaft 202 is inserted at its laterallydistal portion through shaft hole 203 b. Pivot shaft 202 is inserted ata laterally proximal end portion into vehicle frame casing 11 throughouter side surface 11 c and, further, is prevented from being disengagedtherefrom. Namely, plate member 203 is rotatably and pivotally supportedthrough pivot shaft 202 protruding rightwardly and leftwardly in thehorizontal direction from outer side surface 11 c of vehicle framecasing 11. Servo main body 211 of servo portion 210 is secured, at itsmiddle portion in the forward and rearward direction, to plate member203. Thus, the entire servo set 200 (namely, servo portion 210 andneutral returning portion 220) can vertically oscillate at its rear endportion and its front end portion, about pivot shaft 202.

In other words, servo set 200 includes plate member 203 having pivotshaft 202 to be inserted through shaft hole 11 f formed in vehicle framecasing 11, such that plate member 203 is prevented from being disengagedfrom vehicle frame casing 11, and servo main body 211 of servo portion210 is detachably mounted to plate member 203. With this structure, itis possible to easily cope with changes of the specifications of servoset 200, through replacement of plate member 203.

It is further understood by those skilled in the art that the foregoingdescription is a preferred embodiment of the disclosed device and thatvarious changes and modifications may be made in the invention withoutdeparting from the scope thereof defined by the following claims.

What is claimed is:
 1. A control mechanism for a stepless transmissionconnected to a manipulation lever in the stepless transmission anddisposed outside a housing of the stepless transmission, the controlmechanism for the stepless transmission comprising: a) a piston rodconnected to the manipulation lever; b) a piston provided on the pistonrod wherein the piston is coaxial with the piston rod; c) a cylindercase provided with a cylinder configured to house the piston rod and thepiston such that the piston rod and the piston are displaceable in anaxial direction, the piston and the cylinder forming a first fluidchamber to be supplied with a hydraulic fluid for withdrawing the pistonrod from the cylinder and a second fluid chamber to be supplied with thehydraulic fluid for introducing the piston rod into the cylinder; d) aspring configured to bias the manipulation lever in a neutral directioncoaxially with the piston rod; e) a proportional pressure control valveconfigured to selectively supply the hydraulic fluid to the first fluidchamber or the second fluid chamber, the proportional pressure controlvalve being mounted to the cylinder case; and f) a pivot shaft fixed tothe housing and configured to support the cylinder case such that thecylinder case rotates around an axis of the pivot shaft with respect tothe housing.
 2. The control mechanism for the stepless transmissionaccording to claim 1, wherein the proportional pressure control valve iscomposed of a first valve configured to supply the hydraulic fluid tothe first fluid chamber and a second valve configured to supply thehydraulic fluid to the second fluid chamber, and an axis center of thepiston rod, an axis center of displacement of a valve body of the firstvalve, and an axis center of displacement of a valve body of the secondvalve form a triangle shape, when viewed in the axial direction of thepiston rod.
 3. The control mechanism for the stepless transmissionaccording to claim 1, wherein the spring is arranged in the axialdirection such that a coil axis center of the spring and the axis centerof the piston rod are substantially coincident with each other.
 4. Thecontrol mechanism for the stepless transmission according to claim 3,wherein the piston rod, the first valve, and the second valve aredisposed such that the axial direction of the piston rod, a direction ofdisplacement of a valve body of the first valve and a direction ofdisplacement of a valve body of the second valve are parallel with eachother, and the spring is disposed on one end of the cylinder case in theaxial direction of the piston rod, and the first valve and the secondvalve are disposed on the other end of the cylinder case on an oppositeside to the spring, in the axial direction of the piston rod.
 5. Thecontrol mechanism for the stepless transmission according to claim 1,further comprising a mounting part disposed around and secured to thepivot shaft such that the mounting part rotates about the pivot shaftand is prevented from being disengaged from the housing, wherein themounting part is detachably mounted to the cylinder case.
 6. The controlmechanism for the stepless transmission according to claim 5, whereinthe cylinder case is mounted to the mounting part such that an axis lineof the piston rod and an axis line of the pivot shaft are overlappedwith each other in a plan view.
 7. The control mechanism for thestepless transmission according to claim 1, wherein the piston rod isprovided with a thread portion at a tip end portion and a flat portionin at least a portion of an outer peripheral surface of the piston rod,and wherein the piston rod is secured, through screwing, to a mountingpart.